The black discoloration in your nails is most often trapped blood from a minor injury, but it can also come from fungal infections, natural pigmentation, or, in rare cases, something more serious like melanoma. The cause usually depends on whether the dark area appeared suddenly after trauma, developed slowly as a streak, or spread across the whole nail.
Bruising Under the Nail
The most common explanation is a subungual hematoma, which is blood pooling between the nail and the nail bed after an injury. Stubbing your toe, slamming a finger in a door, or wearing tight shoes during a long run can all cause it. The blood gets trapped with nowhere to go, and as it dries it shifts from red or purple to dark brown or black.
One easy way to confirm this is the cause: the dark spot moves forward as your nail grows. Over weeks, you’ll notice more normal-colored nail appearing between the discoloration and your cuticle. The whole process of growing out takes six to nine months, so the stain can stick around for a while even though the injury itself has healed. If you remember banging or jamming the nail, and the dark area is slowly migrating toward the tip, a bruise is almost certainly what you’re looking at.
Fungal Infections
Nail fungus doesn’t always look yellow or white. Certain mold species, particularly types of Aspergillus, produce pigments that turn an infected nail greenish, brown, or outright black. These molds account for roughly 2 to 25 percent of all fungal nail infections and are more common in toenails, especially if your feet spend a lot of time in warm, damp shoes.
A fungal cause usually comes with other changes beyond just color. The nail may thicken, become crumbly or brittle at the edges, lift away from the nail bed, or develop a slightly foul smell. The discoloration tends to spread gradually over weeks to months rather than appearing overnight. If you’re seeing black or dark green patches along with any of those texture changes, a fungal infection is worth investigating. A doctor can clip a small piece of the nail and test it to confirm.
Natural Pigmentation
Dark vertical lines running from cuticle to tip are common in people with deeper skin tones. This is sometimes called racial or constitutional melanonychia, and it’s seen frequently in Black, Asian, Hispanic, and Middle Eastern populations. The lines are caused by melanin deposited into the nail as it forms, and they’re completely harmless. Multiple nails are often affected, which is one clue that the pigment is just part of your body’s normal coloring rather than a sign of disease.
Certain medications can also trigger dark nail bands. Some antiretroviral drugs and chemotherapy agents stimulate the pigment-producing cells at the base of the nail, creating streaks that fade once the medication is stopped.
Splinter Hemorrhages
If you’re seeing thin, hair-like lines of dark red or reddish-brown running vertically under the nail, those are splinter hemorrhages. They look exactly like tiny splinters embedded in the nail bed. Minor trauma is the most common cause, and a single line that appears after physical work or sports is rarely anything to worry about.
When splinter hemorrhages show up on multiple nails without any obvious injury, they can signal problems elsewhere in the body. Infection of the heart valves (endocarditis) is one well-known cause. Inflammation of blood vessels (vasculitis) or tiny blood clots damaging capillaries can also produce them. Multiple unexplained splinter hemorrhages, especially combined with fever, fatigue, or joint pain, are worth getting checked.
When Dark Nail Color Could Be Melanoma
Subungual melanoma is rare, but it’s the one cause of black nails you don’t want to miss. It typically starts as a single dark streak on one nail, usually brown to black, and changes over time. About 65 percent of cases begin as a vertical pigmented band that exceeds 3 millimeters in width and shows irregular or blurry borders. The streak may widen near the cuticle, and the pigment sometimes bleeds onto the surrounding skin at the sides or base of the nail, a warning sign known as Hutchinson’s sign.
Dermatologists use an ABCD checklist to screen for nail melanoma:
- A (Adult age): It develops in adults, not children.
- B (Brown bands, brown background): A dark streak sitting on an overall brownish nail bed, often with uneven color.
- C (Color on surrounding skin): Pigment extending past the nail onto the cuticle or skin around the nail.
- D (One digit): Only a single finger or toe is affected.
Not every dark streak is melanoma, and not every melanoma shows Hutchinson’s sign. But a new, solitary dark band in an adult that is wider than 3 millimeters, changing in size or color, or spreading pigment beyond the nail itself warrants a dermatology visit. A biopsy of the nail matrix is the only definitive way to rule it out.
How to Tell the Causes Apart
A few practical clues can help you narrow things down before you ever see a doctor. A bruise moves with the nail as it grows and usually follows an injury you can remember. Fungal infections change the texture of the nail, not just the color, and tend to worsen slowly. Natural pigmentation often affects more than one nail and has been present for years. Melanoma is typically a single streak on a single nail that evolves over weeks to months.
Color alone isn’t a reliable way to diagnose anything. A bruise, a fungal infection, a benign pigment line, and early melanoma can all look black. What matters more is the pattern: how many nails are involved, whether the discoloration is changing, and whether the nail’s texture or the surrounding skin looks abnormal. If the dark area appeared without trauma, affects only one nail, and is getting wider or darker, getting a professional evaluation is the right next step.