A black spot in the corner of your toenail is almost always a small bruise, technically called a subungual hematoma. Blood from a damaged blood vessel pools beneath the nail plate, creating a dark red, purple, or black mark. Less commonly, the discoloration can come from a fungal or bacterial infection, or, in rare cases, a type of skin cancer called subungual melanoma. The location, shape, and behavior of the dark spot over time can help you tell these apart.
Bruising Under the Nail
The most common explanation is straightforward: something injured the blood vessels in your nail bed, and blood collected in one spot. You might remember stubbing your toe or dropping something on it, but you don’t need a single dramatic injury for this to happen. Repetitive pressure, like running in shoes with a tight toe box, can cause the same thing. The constant contact between the nail and the shoe wall damages tiny blood vessels bit by bit, and the blood pools where the pressure is greatest, often at the corner or edge of the nail.
A bruise under the nail typically looks like a well-defined dark spot, ranging from deep red to nearly black. It doesn’t change shape on its own, but it does migrate forward as the nail grows. Toenails grow at roughly 1.6 mm per month, about half the speed of fingernails, so a bruise near the base of a big toenail can take 9 to 12 months to fully grow out and disappear. If the spot is near the tip, you may see it gone in a few weeks as you trim the nail.
Most of these bruises need no treatment. If the pressure from trapped blood is painful and the injury just happened, a doctor can perform a simple procedure called trephination, where a small hole is made in the nail to let the blood drain. This works best within 24 to 48 hours of the injury. If the nail itself is cracked, split, or separating from the nail bed, the nail plate may need to be removed so the tissue underneath can be examined and repaired.
Fungal and Bacterial Infections
Fungal toenail infections can sometimes produce dark pigment, turning part or all of the nail brown or black. This usually looks different from a bruise: the nail tends to thicken, become brittle, or lift away from the nail bed. The discoloration is often patchy or diffuse rather than a single well-defined spot.
A bacterial infection, particularly one involving Pseudomonas, produces a distinctive green-to-black discoloration. It’s often accompanied by a slightly sweet or fruity odor and may appear in a nail that’s already damaged or partially detached. This type of infection tends to develop in nails that stay moist for long periods or have been previously injured.
When the Spot Could Be Melanoma
Subungual melanoma is rare, but it’s the one cause you shouldn’t overlook. Unlike a bruise, melanoma under the nail typically appears as a dark vertical streak or band running from the base of the nail toward the tip. It doesn’t grow out with the nail. Instead, it stays in place or slowly widens and darkens over time. It most commonly affects the big toe, though it can occur on any digit.
Dermatologists use an ABCDEF framework to evaluate suspicious nail streaks:
- Age: Peak incidence is between the 50s and 70s. It’s also more common in Black, Asian, and Native American populations, where it can account for up to one-third of all melanoma cases.
- Band characteristics: A brown-to-black band 3 mm or wider, with uneven color or irregular borders.
- Change: The band is getting darker, wider, or more irregular over weeks to months.
- Digit: The thumb, index finger, and big toe are highest risk.
- Extension: Pigment spreading from the nail onto the surrounding skin fold, known as the Hutchinson sign. This is one of the strongest indicators of melanoma.
- Family history: A personal or family history of melanoma or atypical moles.
A few additional red flags: the discoloration affects only one nail, it bleeds or ulcerates without trauma, or the nail itself is distorted or crumbling. Any of these warrant a visit to a dermatologist, ideally one who specializes in nail conditions. Fair-skinned individuals with any new dark band on a toenail should also have it evaluated, since longitudinal melanonychia (dark vertical nail lines) is uncommon in lighter skin and more likely to be significant when it appears.
How to Tell the Difference at Home
The single most useful test you can do yourself is to wait and watch. A bruise moves forward with nail growth. Over a month or two, you should see the dark spot shift slightly toward the tip of the nail. A melanoma streak stays anchored in place because the pigment-producing cells sit in the nail matrix at the base, continuously depositing color into the new nail as it forms.
Consider the shape. A bruise is usually a blob or irregular patch with no particular orientation. Melanoma tends to form a line running lengthwise down the nail. Color matters too: a bruise often has a reddish-purple hue, especially when fresh, and gradually fades to brown before growing out. Melanoma streaks stay dark and may contain multiple shades of brown and black within the same band.
Think about context. If you recently started a new running routine, bought tighter shoes, or remember any impact to the toe, a bruise is overwhelmingly likely. If you can’t recall any injury and the mark has been there for more than a few months without moving or fading, that’s a reason to get it looked at.
What to Expect if You Get It Checked
A dermatologist will typically examine the nail with a dermatoscope, a handheld magnifying device with a light that reveals pigment patterns invisible to the naked eye. Under magnification, a bruise shows a homogeneous blob of blood, while melanoma reveals irregular streaks of pigment with varying thickness and spacing.
If the appearance is suspicious, the next step is a biopsy of the nail matrix, the tissue at the base of the nail where new nail cells are produced. This involves numbing the toe, removing part of the nail plate, and taking a small sample of tissue. It’s an outpatient procedure, and while it can temporarily deform the nail as it regrows, it provides a definitive answer. Early detection of nail melanoma significantly improves outcomes, which is why dermatologists take a low threshold for biopsy when the clinical picture is uncertain.