A black fingernail almost always falls into one of a handful of categories: bleeding under the nail from an injury, a pigmented streak produced by melanocytes in the nail matrix, a fungal or bacterial infection, a medication side effect, or, far less commonly, a melanoma growing beneath the nail plate. The most frequent cause by a wide margin is trauma, but because the rare causes include cancer, a black nail that appears without an obvious injury and does not grow out deserves professional evaluation.
Subungual Hematoma, the Most Common Culprit
If you slammed your finger in a door, dropped something heavy on it, or wore tight shoes during a long hike, the dark discoloration you see is almost certainly a subungual hematoma, which is simply a pool of blood trapped between the nail plate and the nail bed. The color can range from deep red to purple to jet black depending on how much blood collected and how long it has been there. In most cases, the discoloration is accompanied by throbbing pain right after the injury, and you can usually recall exactly what happened.
A small hematoma that does not cause significant pain will grow out on its own as the nail advances. Fingernails grow roughly three to four millimeters per month, so a hematoma near the base of the nail can take several months to fully disappear. When the blood pocket is large and the pressure causes intense pain, a doctor can drain it through a procedure called nail trephination, where a small hole is made in the nail plate to release the trapped blood. A controlled-drill technique has been shown to penetrate the nail without breaching the nail bed underneath, providing substantial pain relief within hours and carrying minimal risk of complications.1PubMed. Controlled nail trephination for subungual hematoma
There is one important caveat with hematomas. If you do not remember any injury but still see a dark patch under the nail, do not assume trauma is the explanation. Blood under a nail tracks with the nail as it grows forward, while melanoma pigment typically stays anchored in the same spot relative to the nail matrix. A dermatologist can use dermoscopy to distinguish the two, so when in doubt, get it checked.
Longitudinal Melanonychia and Why a Dark Stripe Matters
A pigmented vertical band running from the base of the nail toward the tip is called longitudinal melanonychia. It looks like a brown, dark brown, or black stripe, and it can appear on one or several nails. The stripe forms because melanocytes in the nail matrix, the tissue under the cuticle where the nail is produced, are depositing melanin into the growing nail plate.
Not every dark streak is dangerous. Most are caused by either melanocytic activation, where the existing melanocytes simply ramp up pigment production without increasing in number, or by a benign nevus (a mole) in the nail matrix. Melanocytic activation tends to deposit scattered melanin granules without forming cell clusters, while a nail matrix nevus shows distinct nests of nevus cells on biopsy.2PubMed Central. Optimizing Biopsy Decisions in Benign Longitudinal Melanonychia: A Dermoscopic-Pathologic Study of Nail Matrix Nevus vs. Melanocytic Activation Both are benign and can be monitored without treatment. The third possibility in this category, subungual melanoma, is the one that demands urgent attention.
Longitudinal melanonychia is more common and often completely normal in people with darker skin tones. Multiple nails showing faint bands simultaneously are usually a sign of racial melanonychia, a benign variant that requires no intervention. A new, solitary band on a single nail that is widening, darkening, or developing uneven coloring over time is the pattern that warrants a prompt dermatology visit.3Chinese Journal of Plastic and Reconstructive Surgery. Longitudinal Melanonychia: How to Distinguish a Malignant Condition from a Benign One
Subungual Melanoma and What to Watch For
Subungual melanoma is a rare form of skin cancer that originates in the nail matrix. It accounts for a small fraction of all melanomas, but it carries outsized consequences because it is often diagnosed late. Unlike a mole on exposed skin, a pigmented lesion hidden under a nail is easy to ignore or dismiss as a bruise. The delay in diagnosis tends to mean that subungual melanoma is caught at a more advanced stage than cutaneous melanomas elsewhere on the body.
One clinical sign clinicians look for is the Hutchinson sign, where pigment from the nail bed extends onto the surrounding skin of the cuticle area or the fingertip. When present, it strongly raises suspicion for melanoma. In one study of cases showing the Hutchinson sign, almost all displayed the pigment on the hyponychium (the skin under the free edge of the nail), and dermoscopic examination showed irregular diffuse pigmentation in every case.4PubMed. Hutchinson Sign: Biopsy May Assist in Diagnosis of Subungual Melanoma in Situ
However, the Hutchinson sign is not as reliable as it was once considered. A scoping review of dermoscopic features in nail unit melanoma found that the Hutchinson sign appeared in fewer than half of patients, meaning the majority of subungual melanomas did not display this classic warning sign.5PubMed. Dermoscopic features of nail unit melanoma: a scoping review Furthermore, benign conditions like congenital nail nevi can produce a “pseudo-Hutchinson sign” that mimics the real thing, so pigment extending beyond the nail does not automatically mean cancer.6PubMed. Congenital subungual melanocytic nevus with a pseudo-Hutchinson sign The features most reliably associated with nail melanoma on dermoscopy were instead color variation within the band, irregular spacing and width of the pigmented lines, and blurred borders.
What this means for you practically: a single dark stripe that is changing over weeks or months, especially on the thumb or big toe, especially if the band is wider than about three millimeters or if the nail plate is starting to crack or become dystrophic, deserves a dermatology evaluation. Do not wait for the Hutchinson sign to appear before seeking assessment. Invasive melanoma was more frequently associated with destructive changes like nail plate dystrophy and ulceration, while melanoma in situ tended to show subtler pigment abnormalities.5PubMed. Dermoscopic features of nail unit melanoma: a scoping review
Infections That Turn Nails Dark
Bacterial and fungal infections can produce nail discoloration that ranges from green to greenish-black to true black. The most recognizable is green nail syndrome, caused by Pseudomonas aeruginosa, a bacterium that thrives in moist environments. People whose hands are constantly in water, or who have pre-existing nail damage that allows moisture to pool under the nail, are at highest risk. The discoloration results from pigments the bacteria produce and can shift from greenish-yellow to greenish-black depending on the severity.7PubMed Central. Chloronychia: green nail syndrome caused by Pseudomonas aeruginosa in elderly persons Treatment typically involves an oral antibiotic, and keeping the nails dry is the most important preventive step.
Fungal nail infections, or onychomycosis, more commonly turn nails yellow, white, or brownish, but some molds can produce dark brown or black discoloration. These infections develop slowly and are usually accompanied by thickening and crumbling of the nail plate rather than the sudden onset you would expect from a hematoma. If you have a thickened, discolored nail that has been getting worse over months, a fungal culture or clipping analysis can confirm the diagnosis. Unlike bacterial infections, fungal nail infections tend to require prolonged treatment with oral antifungals.
Medications That Can Darken Nails
A number of drugs are known to cause nail pigmentation as a side effect. Chemotherapy agents are among the most frequent offenders. Patients undergoing cancer treatment often develop nail changes including darkening, and while the discoloration can be alarming, it is usually cosmetic and resolves after the drug is stopped.8PubMed Central. Nail Changes With Chemotherapeutic Agents and Targeted Therapies
Hydroxyurea, used for conditions like polycythemia vera and sickle cell disease, is another drug that can cause melanonychia. In one reported case, a patient developed nail darkening as a late-onset side effect, well after starting the medication, which is a pattern that can easily be mistaken for a new medical problem rather than a drug reaction.9PubMed Central. Late-Onset Hydroxyurea-Induced Melanonychia and Tongue Hyperpigmentation in a Patient With Polycythemia Vera: A Case Report Antimalarials, certain antibiotics, and antiretroviral drugs have also been linked to nail pigmentation. If you notice your nails darkening after starting a new medication, mention it to your prescribing doctor. In most cases the change is harmless and reversible, but confirmation avoids unnecessary worry.
How Dermatologists Evaluate a Black Nail
When you show up with a concerning nail, the first tool a dermatologist reaches for is a dermatoscope, a handheld magnifying device with a built-in light. Nail dermoscopy, sometimes called onychoscopy, lets the clinician see through the nail plate and evaluate the pigment pattern underneath. It can distinguish between melanin-based pigment and blood, identify irregular line patterns that suggest melanoma, and support the diagnosis of nonmalignant conditions like fungal infections or psoriasis.10Journal of Dermatology for Physician Assistants. Overview of onychoscopy: Dermoscopy for the nail Routine use of dermoscopy in evaluating pigmented nails has been shown to help avoid unnecessary biopsies while improving the accuracy of identifying cases that need one.11PubMed Central. Use of Nail Dermoscopy in the Management of Melanonychia: Review
If the dermoscopic findings are ambiguous or suspicious, the next step is a nail matrix biopsy. This is a more involved procedure than a standard skin biopsy because the tissue of interest sits under or at the root of the nail. Dermatologists have several techniques at their disposal, including a small punch excision, a lateral longitudinal excision, and a shave biopsy of the matrix, each suited to different sizes and locations of the pigmented area.12PubMed. Nail matrix biopsy of longitudinal melanonychia: diagnostic algorithm including the matrix shave biopsy A newer approach uses a CO2 laser to create a small window in the nail plate, allowing the surgeon to visualize the matrix pigmentation directly with intraoperative dermoscopy before taking a targeted sample. In a series of cases, this technique correctly diagnosed melanoma in situ and benign activation without significant complications.13PubMed. A Novel Window Technique Using CO2 Laser, and a Review of Methods for Nail Matrix Biopsy of Longitudinal Melanonychia
The goal of this staged approach, dermoscopy first and biopsy when needed, is to balance caution against unnecessary procedures. A nail matrix biopsy can sometimes cause a permanent ridge or cosmetic change in the nail, so clinicians try to reserve it for cases where there is a meaningful possibility of melanoma.
When Children Have Black Nails
Dark nail pigmentation in children causes considerable parental anxiety, but the picture is overwhelmingly reassuring. Brown, gray, and black pigmentation of children’s nails has many causes, almost all of which are benign and require no treatment. Melanoma is the diagnosis no one wants to miss, but it is extremely rare in the pediatric population.14PubMed Central. Nail discoloration in pediatric skin of color patients The most common cause of dark bands in children’s nails is benign melanocytic activation, and in children with darker skin tones, multiple pigmented bands across several nails are a normal finding.
That said, an isolated darkening band on a single nail in a child that is rapidly changing or widening should still be evaluated. The rarity of pediatric melanoma does not mean it is impossible, and the same dermoscopic criteria that apply to adults can help guide assessment in children. Most pediatric dermatologists prefer to monitor stable, uniform bands with periodic photographs rather than jumping to a biopsy, since the likelihood of malignancy is so low.
Practical Signals for When to See a Doctor
Not every black nail requires medical attention. If you remember the injury, the discoloration matches the site of impact, and the dark spot is slowly growing outward with the nail, you are almost certainly watching a hematoma resolve on its own. Similarly, if multiple nails have faint, symmetric brownish bands that have been there for years and are not changing, there is generally nothing concerning going on.
The situations that do call for evaluation include:
- No clear injury: A dark spot or band that appeared without any trauma you can recall.
- Changing pigment: A band that is widening, darkening, developing uneven color, or showing blurred edges over weeks to months.
- Pigment beyond the nail: Darkening of the skin around the cuticle or at the fingertip, even if the nail itself looks only mildly pigmented.
- Nail destruction: Cracking, splitting, or lifting of the nail plate along with dark pigment.
- Single nail involvement: One affected nail, especially the thumb or great toe, warrants more scrutiny than symmetric changes across several nails.
- Persistent discoloration on medication: A new dark nail while taking chemotherapy or other pigment-altering drugs should be confirmed as drug-related rather than assumed to be.
When in doubt, a brief visit for dermoscopy is low-burden and high-yield. Most of the time the answer will be reassuring, and on the rare occasions it is not, early detection makes a significant difference in outcomes for subungual melanoma.
The Emotional Side of Nail Discoloration
It is easy to focus entirely on the medical side and overlook how much a visibly abnormal nail affects daily life. Research suggests the psychological burden can be more impactful than the physical symptoms. A cross-sectional study of Korean adults found that appearance-related nail stress was significantly associated with lower self-esteem and lower health-related quality of life, and that these psychological effects accounted for a meaningful share of the overall impact on wellbeing, more so than physical discomfort alone.15PubMed. Appearance-based nail stress, self-esteem, and health-related quality of life: a cross-sectional study in Korean adults People reported reduced social participation and stigma-related experiences tied to how their nails looked.
If a dark nail is making you self-conscious, know that cosmetic camouflage options exist, including specialized nail polishes and nail wraps designed for discolored nails. For drug-induced discoloration that will eventually resolve, simply knowing that the change is temporary and harmless can go a long way. And for anyone dealing with a prolonged diagnostic workup for a suspicious band, acknowledging the anxiety and seeking support is entirely reasonable. The worry that something could be melanoma, even while waiting for a biopsy result, is stressful in a way that the clinical process does not always account for.