A black nail is almost always caused by trapped blood beneath the nail plate after an injury, a condition called a subungual hematoma. That said, the list of other possibilities is longer than most people expect, ranging from harmless staining by hair dye to a rare but dangerous melanoma hiding under the nail. The color alone does not tell you the cause, so understanding a few distinguishing features can help you decide whether you can wait it out or need to see a doctor promptly.
Trauma and Subungual Hematoma
The most common reason a nail turns black is blunt trauma. Stubbing a toe, slamming a finger in a door, or repetitive pressure from tight shoes during running can all rupture tiny blood vessels in the nail bed. Blood pools underneath the hard nail plate with nowhere to go, and the trapped blood darkens from red to purple to black over a day or two. The pain can be intense because the blood creates pressure in a confined space.
Sometimes, though, people never notice the injury. A case report describes a 61-year-old man with diabetic neuropathy who presented with a completely black toenail. Because he had lost sensation in his feet, the original trauma went unnoticed, and he only sought help after seeing the color change.1PubMed Central. Subungual Hematoma: Insights From a Clinical Case Study That is an important reminder: if you have reduced feeling in your hands or feet from diabetes or another condition, inspect your nails regularly rather than relying on pain to alert you.
When pressure from a hematoma is painful, a doctor can relieve it through a procedure called trephination, which involves making a small hole in the nail plate to let the blood drain. One approach uses a specially designed drill that penetrates the nail without touching the sensitive nail bed, providing substantial pain relief within hours.2PubMed. Controlled nail trephination for subungual hematoma Another common method uses an electrocautery device to melt through the nail. If you wear acrylic nails, mention that to your doctor. A study testing electrocautery on acrylic nails found that roughly four in ten ignited during the procedure, which led researchers to recommend avoiding that technique when acrylic nails are present.3PubMed Central. Up in Flames: The Safety of Electrocautery Trephination of Subungual Hematomas with Acrylic Nails
Most subungual hematomas grow out with the nail over weeks to months. As the nail grows, the dark spot migrates toward the free edge and eventually gets clipped away. If the discoloration does not move with the nail, that is a red flag worth investigating.
Benign Melanonychia
Not every dark streak on a nail means something is wrong. Melanonychia refers to brownish-black discoloration of the nail plate caused by increased melanin production in the nail matrix, the tissue at the base of the nail that generates new nail cells. This often shows up as a vertical band running from the base of the nail to its tip.4PubMed Central. Melanonychia: Etiology, Diagnosis, and Treatment Melanocytes sit in the lower layers of the nail matrix, and when they become more active or more numerous, they deposit extra pigment into the nail plate as it forms.5Archives of Dermatology. Melanocytes of Nail Matrix and Nail Pigmentation
Benign longitudinal melanonychia is especially common in people with darker skin tones. In fact, it is considered a normal finding in many Black, Asian, and Hispanic individuals, where multiple nails may show faint to prominent dark bands without any underlying disease. Research confirms that darker-skinned patients tend to have wider, darker bands on dermoscopy and are also more likely to be sent for biopsy, even when the bands are benign.6PubMed. Clinical and dermoscopic findings of benign longitudinal melanonychia due to melanocytic activation differ by skin type and predict likelihood of nail matrix biopsy That disparity means that if you have darker skin and are told you need a biopsy for a nail band, it is worth asking whether the recommendation is based on genuinely suspicious features or on band width and color alone.
When to Worry About Melanoma
Subungual melanoma is the scenario everyone fears when they see a black nail, and rightly so. It is rare overall, but it accounts for a disproportionately large share of melanomas in people with darker skin. Clinically, it most often presents as a dark band on a single nail, and it can be accompanied by nail splitting or nail-bed bleeding.7PubMed Central. Missed opportunity to diagnose subungual melanoma: potential pitfalls! The peak incidence occurs from the fifth to seventh decades of life, and the thumb and big toe are the most commonly affected digits.8PubMed. The ABC rule for clinical detection of subungual melanoma
One of the most talked-about warning signs is Hutchinson’s sign: dark pigmentation that extends beyond the nail plate onto the surrounding skin folds. First described in 1886, it has long been considered strongly suggestive of subungual melanoma, and dermatologists generally recommend biopsy whenever it is present.9Actas Dermo-Sifiliográficas. Is Hutchinson’s Sign Pathognomic of Subungual Melanoma? But Hutchinson’s sign is not a guarantee of cancer. Pigmentation spilling onto the nail fold also appears in a variety of benign conditions and even in one non-melanoma skin cancer, Bowen’s disease of the nail unit. Sometimes the pigment in the nail bed simply shines through the translucent nail fold, creating a “pseudo-Hutchinson’s sign” that can mislead even experienced doctors.10PubMed. Hutchinson’s sign: a reappraisal
Researchers have proposed an ABCDEF checklist for subungual melanoma. In this system, A refers to the age group most affected and the higher-risk populations (African Americans, Asians, Native Americans), B to brown-to-black bands wider than about 3 mm with irregular borders, C to a change in the band or a lack of improvement despite treatment, D to the digit most commonly involved, E to extension of pigment onto the nail fold, and F to a family or personal history of melanoma.8PubMed. The ABC rule for clinical detection of subungual melanoma The checklist is a useful memory aid, but its diagnostic accuracy has limits. A retrospective study found that the number of ABCDEF criteria met did not reliably distinguish malignant from benign pigmented nail lesions.11PubMed. Retrospective single-center study evaluating clinical and dermoscopic features of longitudinal melanonychia, ABCDEF criteria, and risk of malignancy In a separate study, applying the ABCDEF rule did not improve dermatologists’ accuracy in diagnosing nail melanoma compared to straightforward clinical examination.12JAMA Dermatology. Dermatologists’ Accuracy in Early Diagnosis of Melanoma of the Nail Matrix The takeaway: checklists are helpful for raising suspicion, but they do not replace a biopsy when a lesion looks concerning.
Practical Warning Signs Worth Knowing
Since no single feature clinches the diagnosis, here is a practical list of features that should prompt a visit to a dermatologist sooner rather than later:
- New dark band on one nail: Especially after age 40, a solitary longitudinal band that was not there before warrants evaluation.
- Band is widening or darkening: Any change over weeks to months is more worrisome than a stable band.
- Pigment spreading beyond the nail: Dark color extending onto the cuticle or surrounding skin (Hutchinson’s sign) needs a biopsy, even knowing that benign mimics exist.
- No clear history of injury: A hematoma that you cannot trace to a specific incident is suspicious.
- Discoloration that does not grow out: A hematoma migrates toward the tip as the nail grows. A band that stays anchored at the base suggests ongoing pigment production.
- Nail plate destruction or ulceration: A nail that is crumbling, splitting, or developing a non-healing sore on the nail bed needs urgent attention.
If you have even one of these features and cannot confidently attribute the color to a recent injury, get it checked. Subungual melanoma is treatable when caught early, and delays in diagnosis are a recurring problem documented in case reports.7PubMed Central. Missed opportunity to diagnose subungual melanoma: potential pitfalls!
Infections That Turn Nails Dark
Fungal nail infections are typically associated with thick, yellowish, crumbly nails, but certain fungi produce pigment that can make a nail look black. Fungal melanonychia can present as a solitary black vertical band that is difficult to distinguish from melanocytic melanonychia on appearance alone.13PubMed Central. Fungal Melanonychia as a Solitary Black Linear Vertical Nail Plate Streak: Case Report and Literature Review of Candida-Associated Longitudinal Melanonychia Striata At least 21 species of dark-pigmented fungi and several non-pigmented species have been documented as causes.14PubMed. Fungal melanonychia Nail clippings sent for fungal culture or microscopy can usually sort this out without a more invasive biopsy.
Bacterial infections can also darken a nail. Pseudomonas aeruginosa, a bacterium that thrives in wet environments, is notorious for producing a green pigment that can range from pale green to greenish-black. This condition, called chloronychia or green nail syndrome, typically affects people whose hands spend a lot of time in water or who have had prior nail damage that allowed the bacteria to colonize the space between the nail plate and bed.15PubMed Central. Chloronychia: green nail syndrome caused by Pseudomonas aeruginosa in elderly persons The color can deepen to near-black in advanced cases.16PubMed. Green nail: Etiology and treatment of chloronychia Treatment with an oral antibiotic and keeping the nail dry usually resolves it.
Medications and Systemic Conditions
Certain drugs can activate melanocytes in the nail matrix as a side effect, producing dark bands or diffuse darkening across several nails at once. Chemotherapy agents are among the most well-known offenders. A case report documents a woman who developed dark pigmentation in her thumbnail during treatment with a combination chemotherapy regimen for breast cancer; cyclophosphamide, docetaxel, and 5-fluorouracil were all implicated.17PubMed Central. Chemotherapy-related striate melanonychia: a case report Other medications linked to melanonychia include certain antiretroviral drugs, antimalarials, and some antibiotics. If nail darkening starts soon after beginning a new medication, that timing is an important clue.
Systemic conditions can trigger nail pigmentation too. Vitamin B12 deficiency is a notable cause, producing a pattern sometimes called Addisonian pigmentation that includes brownish-black hyperpigmentation of the nails, knuckles, palmar creases, and sun-exposed skin.18Pigment International. Pigmentary changes associated with endocrine and metabolic disorders The pigmentation from B12 deficiency is typically reversible with supplementation. Adrenal insufficiency (Addison’s disease), Cushing’s syndrome, and thyroid disorders can produce similar pigmentary changes.19PubMed Central. Addisonian Pigmentation – The Great Mimicker – A Review When multiple nails darken at the same time, especially alongside skin changes elsewhere on the body, the cause is more likely systemic than local.
External Staining and Other Mimics
Before worrying about anything internal, consider what your nails have been exposed to externally. Nail pigmentation can be exogenous, caused by contact with chemicals like silver nitrate, potassium permanganate, tobacco, or hair dyes.20PubMed Central. Through a Nail Darkly Friction from tight shoes can also create marks that mimic true pigmentation. The distinguishing feature of external staining is that the pigment sits on the nail surface rather than within or beneath it. A dermatologist can usually identify this quickly with a close look or by using dermoscopy. Acetone or gentle scraping sometimes removes superficial stains, which rules out an endogenous cause immediately.
Bowen’s disease of the nail unit, a form of squamous cell carcinoma in situ, is another mimic worth mentioning. It can present as a black nail streak that closely resembles melanoma. Some cases are linked to human papillomavirus infection, and because the clinical presentation overlaps so heavily with subungual melanoma, biopsy is often the only way to tell them apart.21PubMed. Surface material analysis for human papillomavirus detection in nail Bowen’s disease caused by HPV type 58
How Doctors Tell the Difference
When you show up with a dark nail, a dermatologist’s first tool is a careful history. Did you injure the nail? When did the color first appear? Has it changed? Are you on any medications? Are other nails affected? These questions alone narrow the possibilities considerably.
Dermoscopy, a technique using a handheld magnifying device with polarized light, is the next step. Under dermoscopy, a subungual hematoma typically shows a homogeneous purple-black pattern, often with peripheral fading as the blood diffuses outward. In a study of hematomas, a homogeneous color pattern was seen in over nine out of ten cases, and features like irregular longitudinal bands, Hutchinson’s sign, and ulceration were absent.22British Journal of Dermatology. Dermoscopy of subungual haemorrhage: its usefulness in differential diagnosis from nail‐unit melanoma By contrast, all 16 melanoma cases in that same study showed Hutchinson’s sign, and the vast majority displayed irregular bands and ulceration. Dermoscopy is not infallible, though. Researchers have documented that hematomas can occasionally show a blue-white color pattern typically associated with melanoma, reinforcing that follow-up observation is important even when the initial impression points toward a hematoma.23PubMed Central. Dermoscopic diagnosis of subunit hematoma: new observations
If dermoscopy and history leave any doubt, biopsy is the definitive step. For suspected melanoma, a full-thickness tissue sample from the pigmented area of the nail matrix is taken. The wedge of tissue must extend down to the bone so that both the depth and spread of any melanoma can be accurately assessed.24Journal of Hand Surgery. Pigmented Nail Lesions: When to Observe, When to Biopsy, When to Widely Excise, and When to Amputate? A longitudinal excision technique is generally preferred because it provides the clearest tissue sample for the pathologist while carrying a low risk of permanent nail deformity when done correctly.25PubMed. Subungual Melanocytic Lesions: Key Clinical and Pathologic Concepts and Biopsy Techniques For suspected fungal infections, simple nail clippings sent for culture are usually enough.
Black Nails in Children
Melanonychia does occur in children, though it is rare. In pediatric patients, dark nail bands are overwhelmingly benign, but they still require careful monitoring because malignant transformation, while extremely uncommon in kids, has been reported. The general approach is regular follow-up visits with photographic documentation, reserving biopsy for cases where features change in a concerning way.26PubMed Central. Pediatric Nail Disorders Parents who notice a dark band on their child’s nail should bring it to a pediatric dermatologist’s attention, but panic is rarely warranted.
Emerging Tools for Nail Diagnosis
Diagnosing pigmented nail lesions remains one of the harder problems in dermatology, partly because even specialists find it tricky to distinguish benign melanonychia from early melanoma on clinical appearance alone. Researchers are now developing artificial intelligence systems trained on large sets of nail images to detect and segment melanonychia lesions. One recent deep-learning model combined object detection with image segmentation and achieved performance closely matching dermatoscopic assessments by trained dermatologists.27PubMed. Establishment of an intelligent analysis system for clinical image features of melanonychia based on deep learning image segmentation These tools are still in the research phase and not yet standard in clinics, but they signal a future where smartphone-quality images could help flag concerning nail lesions earlier, particularly for patients in areas without easy access to dermatologists.