Black discoloration beneath a fingernail or toenail is medically known as melanonychia when caused by pigment. This change can signify a wide range of underlying issues, spanning from common, harmless injuries to rare but serious medical conditions. Determining the origin of the black mark is the first step in deciding whether it requires simple observation or urgent medical attention.
The Immediate Cause: Injury and Blood Clots
The most frequent reason for a black nail is direct trauma, which leads to a condition called a subungual hematoma. This occurs when a crushing injury, such as slamming a finger in a door or dropping a heavy object, damages the tiny blood vessels beneath the nail plate. The resulting bleeding pools in the confined space between the nail and the nail bed, creating intense pressure and throbbing pain.
The blood trapped under the nail quickly darkens from red to deep purple, brown, or black as it dries and clots. This is why the injury is often mistaken for a bruise on the nail itself. A subungual hematoma typically appears as a solid area of discoloration with defined edges, corresponding exactly to the site of the impact, and heals as the new nail grows out gradually from the base.
A fingernail may take six to nine months to completely replace itself, while a toenail can take up to a year, meaning the black spot will persist for many months as it moves toward the tip. Minor hematomas that cause little pain can be monitored at home, but severe or large ones can cause significant discomfort due to the pressure. In such cases, a procedure called trephination can be performed by a healthcare provider to drain the trapped blood and immediately relieve the pressure and pain.
Non-Traumatic Causes: Pigmentation and Infection
Black discoloration that appears without a history of acute injury often stems from the accumulation of pigment or infectious debris within the nail plate. One common source is longitudinal melanonychia, which presents as a brown or black streak running vertically down the nail. This linear pigmentation results from the melanocytes, the pigment-producing cells in the nail matrix, becoming activated and depositing melanin into the growing nail.
In people with darker skin tones, this type of linear streak is a common and usually benign finding, sometimes referred to as ethnic melanonychia. Pigmentation can also be triggered by various medications, including certain chemotherapy agents or antimalarial drugs, or by chronic micro-trauma from repetitive activities or poorly fitting shoes. Additionally, some fungal infections, known as onychomycosis, can cause debris and pigment to build up under the nail, leading to dark, often irregular, discoloration and thickening of the nail plate.
These non-traumatic causes are distinct from hematomas because they do not involve acute pain or swelling and the discoloration often manifests as a stripe rather than a solid, defined bruise. The pattern of the discoloration—whether a single line, multiple bands, or a diffuse stain—provides important clues to its underlying cause.
The Serious Concern: Subungual Melanoma
The most serious, though rare, cause of a black nail is subungual melanoma, a form of skin cancer that originates in the nail matrix. This malignancy usually appears as a dark, irregular longitudinal band that may change in width, color, or shape over months or years. Unlike a bruise, which grows out with the nail, the pigment from a melanoma persists and may worsen in appearance.
A particularly concerning sign is Hutchinson’s sign, where the dark pigment extends beyond the nail plate onto the surrounding skin of the cuticle or nail folds. Although not exclusively linked to melanoma, its presence warrants immediate medical evaluation because it suggests pigment-producing cells are spreading beyond the nail unit. Other features suggesting melanoma include a band that is unusually wide (typically over three millimeters) or one that shows varying shades of color.
The nail plate itself may begin to split, crack, or become distorted as the tumor grows beneath it. Early detection of subungual melanoma is important; though rare, it can be aggressive due to delayed diagnosis. Any dark spot or streak that appears without injury and exhibits these atypical features should be investigated by a specialist.
Treatment and When to See a Doctor
The necessity of medical intervention depends entirely on the cause of the black discoloration. For a subungual hematoma, home care involving rest, ice, and elevation is sufficient for small, non-painful injuries. If the pain is severe or the blood covers more than half of the nail, a healthcare provider should be seen within 48 hours for potential drainage to relieve the painful pressure.
Any black discoloration that appears without a clear traumatic cause should be evaluated by a dermatologist for a definitive diagnosis. This is especially true for any dark streak that involves the cuticle or surrounding skin, as indicated by Hutchinson’s sign, or one that is rapidly changing or widening. A physician can perform a biopsy of the nail matrix to differentiate between benign melanonychia, drug-induced pigmentation, or subungual melanoma.
If the cause is a fungal infection, treatment focuses on oral or topical antifungal medications to clear the underlying pathogen. Any persistent black mark that fails to grow out with the nail or is accompanied by changes in shape or texture requires a medical consultation for early diagnosis and treatment.