Black toenails are almost always caused by blood trapped under the nail after some kind of injury. A single hard impact or weeks of repetitive pressure can burst tiny blood vessels in the nail bed, and because there’s no space between the nail plate and the tissue beneath it, the blood pools and turns dark red, purple, or black. Less commonly, black discoloration comes from a fungal infection, a medication side effect, or, rarely, a type of skin cancer that develops under the nail.
Trauma and Bruising Under the Nail
The most common explanation is a subungual hematoma, which is simply a bruise beneath the nail. A heavy object dropped on your toe, stubbing it hard against furniture, or slamming it in a door can all rupture blood vessels in the nail bed. The trapped blood has nowhere to drain, so it creates visible dark discoloration and often significant throbbing pain. In many cases, though, the injury is minor enough that you don’t remember it happening, which is why a black toenail can seem to appear out of nowhere.
Repetitive, low-grade trauma is just as common as a single hard hit. Runners and hikers are especially prone to this. Every stride pushes your toes against the front or side of your shoe, and over hundreds or thousands of steps, that repeated contact damages blood vessels beneath the nail. Running downhill makes it worse because gravity shifts your foot forward inside the shoe. This is common enough that it has its own nickname: runner’s toe.
Footwear is the biggest controllable factor. Shoes with a too-small toe box concentrate pressure on the big toe and second toe, while shoes that are too loose let the foot slide forward with each step. Getting properly sized at a running store, keeping laces snug enough to prevent sliding, and wearing moisture-wicking socks that reduce friction all help prevent the problem.
What a Bruised Nail Looks Like Over Time
A fresh subungual hematoma is usually dark red or purplish and painful to the touch. Over the following days it turns darker, eventually appearing black. The discoloration stays put as the nail grows out, so you’ll watch the dark spot slowly migrate toward the tip of your toe over the course of several months. Toenails grow at roughly 1.6 mm per month, which is less than half the speed of fingernails. A full toenail replacement takes about 12 to 18 months, so a black mark near the base of the nail can stick around for a long time.
Applying ice wrapped in a towel shortly after the injury is often enough for mild cases. But if the pain is severe or the dark area is growing, a healthcare provider can drain the blood through a small hole in the nail plate. This procedure, called trephination, works best within 24 to 48 hours of the injury and provides almost immediate pressure relief. If the nail is cracked, split, or lifting away from the nail bed, more involved treatment may be necessary, sometimes including removal of the nail plate. Attempting to drain the blood yourself risks infection and further damage.
Fungal Infections
Fungal nail infections typically turn nails yellow, white, or brownish, but in some cases they can produce very dark discoloration that looks black. The nail usually thickens, becomes brittle, and may start to separate from the nail bed. Unlike a bruise, a fungal infection won’t grow out on its own and tends to gradually worsen. The affected nail often has a crumbly or chalky texture rather than a smooth surface with a dark spot underneath. Fungal infections are more common in toenails than fingernails because feet spend more time in warm, moist environments like shoes.
Medications That Darken Nails
Certain drugs can trigger nail pigmentation by activating the pigment-producing cells in the nail matrix. Chemotherapy agents are the most well-known culprits. The effect can show up as one or more brown-to-black vertical bands running the length of the nail, or as diffuse darkening of the entire nail plate. If you’ve recently started a new medication and notice your nails changing color, your prescriber can help determine whether the drug is responsible.
Nail Changes From Chronic Illness
Systemic diseases can alter nail appearance in subtle ways. In one study of 70 patients with chronic kidney failure, 80% had visible nail abnormalities. These included dark longitudinal streaks (splinter hemorrhages) in nearly 59% of patients and pigmented bands in about 14%. A pattern called “half and half nails,” where the lower portion of the nail appears pale and the upper portion turns reddish-brown, showed up in roughly a third of patients. These changes can also appear in conditions affecting blood clotting or circulation. On their own, nail changes don’t diagnose a systemic illness, but they can be one piece of a larger picture.
When Black Means Melanoma
Subungual melanoma is a rare but serious form of skin cancer that develops in the nail bed. It accounts for a small fraction of all melanomas, but it’s disproportionately common in people with darker skin tones, including Black, Asian, and Native American populations. The key visual difference from a bruise: melanoma typically appears as a dark vertical streak or band running from the base of the nail to the tip, and it does not grow out with the nail over time. Instead, the band stays in place and may slowly widen, darken, or develop uneven borders.
Clinicians use a set of warning signs sometimes called the ABCDEF criteria to evaluate suspicious nail pigmentation:
- Age and ancestry: Most common between ages 50 and 70, and in African American, Native American, or Asian individuals
- Band appearance: Black or brown color, 3 mm or wider, with blurred or irregular borders
- Change: The band changes in size, shape, or color over time, or doesn’t improve with appropriate treatment
- Digit: The thumb and big toe are most commonly affected, and involvement of a single nail is more concerning than multiple nails
- Extension: Pigment spreading into the skin around the nail (called the Hutchinson sign) is highly suggestive of melanoma
- Family history: A personal or family history of melanoma raises risk
The Hutchinson sign deserves special attention. When dark pigment extends beyond the nail plate into the surrounding skin fold or cuticle, it signals that pigment-producing cells are spreading outward. Sometimes this pigment spread is only visible under magnification, but even that microscopic version is a strong indicator. Subungual melanoma grows slowly, sometimes taking months before visible changes become obvious, which is why a dark band that persists without change or explanation warrants evaluation.
How to Tell the Difference at Home
A bruise from trauma is usually a round or irregularly shaped blotch that sits in one area of the nail and migrates toward the tip as the nail grows. You can often trace it back to a specific injury or a period of intense physical activity. A melanoma streak, by contrast, runs vertically from the cuticle to the free edge, stays in place rather than growing out, and may gradually become wider or more irregular.
If your toenail turned black after a clear injury and the dark spot is slowly moving toward the tip, you’re most likely watching a bruise grow out. If a dark band appeared without any injury you can recall, hasn’t moved in several months, is getting wider, or is accompanied by pigment spreading into the surrounding skin, those are reasons to have it looked at promptly. A nail biopsy is a straightforward procedure that provides a definitive answer.