A black toenail often comes from trapped blood after trauma, though fungus, pigment changes, and melanoma can also be behind it.
A black toenail can feel alarming, especially when it seems to show up out of nowhere. In many cases, the cause is plain old trauma. A stubbed toe, a tight shoe, or miles of downhill running can leave blood trapped under the nail. That dark mark may sit there for weeks, then slowly move forward as the nail grows out.
Still, not every black toenail is a bruise. Nail fungus can darken and thicken a toenail. Natural pigment can create a brown or black streak. A rare skin cancer called subungual melanoma can also start under a nail. The trick is knowing which pattern fits what you’re seeing.
Why A Toenail Can Change To Black
Toenails are small, but they take a beating. Pressure, impact, sweat, friction, and moisture all pile up down there. When the nail plate or the skin under it gets injured or inflamed, color can shift from red to purple to dark brown or black.
The shade matters. So does the shape. A blotchy patch after a run points in one direction. A narrow dark streak that widens over time points in another. Pain, swelling, thickening, or drainage can also change the picture.
Bruising Under The Nail
This is the most common reason. The medical name is subungual hematoma. Blood gets trapped under the nail after impact or repeated rubbing. You might notice throbbing pain at first, then a dark patch that turns almost black. After the soreness fades, the color usually stays until the nail grows out.
Runners, hikers, soccer players, and people who wear cramped shoes see this a lot. If the dark spot starts near the front edge and slowly marches outward over months, trauma is high on the list.
Fungal Infection
Fungus does not always make a nail yellow. It can also leave a nail brown or black, especially when debris builds up under the plate. A fungal nail often gets thicker, rougher, and more brittle at the same time. It may crumble at the edge or start lifting from the skin below.
This pattern tends to creep in, not show up overnight. If the nail has looked dull, thick, or misshapen for a while, fungus becomes more likely than a bruise.
Pigment Changes In The Nail
Some people develop brown or black pigment lines in a nail. This is called melanonychia. It can happen from repeated friction, pregnancy, certain medicines, skin tone, or nail matrix activation. These streaks may be harmless, but a new or changing dark band still deserves a careful check.
Melanoma Under The Nail
Subungual melanoma is rare, but it matters because it can look like an innocent dark mark. It often shows up as a brown or black streak, patch, or uneven pigment that does not grow out like a bruise. The color may spread into the skin around the nail, and the nail may split, lift, or distort.
If there was no injury and the spot keeps changing, don’t shrug it off. A dermatologist or podiatrist can sort out the pattern far better than guesswork at home.
Can Tight Shoes Cause A Black Toenail Over Time?
Yes. Tight shoes are a classic trigger. Each step bangs the nail against the shoe box. That repeated hit can break tiny blood vessels under the nail. Long runs, steep descents, sudden stops in court sports, or boots with a short toe box all raise the odds.
The nail does not need one dramatic injury. Small hits over and over can do the same thing. That’s why many people first spot the black color after a trip, race, tournament, or a busy workweek spent on their feet.
Signs The Shoe Fit Is Part Of The Problem
- Your second toe or big toe keeps rubbing the front of the shoe.
- You feel pressure when going downhill or braking hard.
- The dark mark shows up after long walks or runs.
- More than one toenail starts looking bruised.
- The issue keeps coming back with the same pair.
If that sounds familiar, the nail may heal, but the cycle can repeat until the shoe fit changes.
| Possible Cause | How It Usually Looks | What Tends To Go With It |
|---|---|---|
| Subungual hematoma | Dark red, purple, or black patch under the nail | Pain after stubbing the toe, running, or shoe pressure |
| Tight shoes or sports friction | Black area near the front or middle of the nail | Shows up after long activity, may affect more than one toe |
| Fungal infection | Brown, black, or yellow nail with rough texture | Thickening, crumbling, lifting, slow change |
| Melanonychia | Brown or black vertical streak | May be stable, may involve one or several nails |
| Subungual melanoma | Dark streak or patch that changes over time | New pigment, widening band, nail distortion, color on nearby skin |
| Nail psoriasis | Discolored nail with pits or oil-like spots | Thick nail, lifting, skin rash elsewhere |
| Bacterial debris or old blood | Dark build-up under a partly lifted nail | Odor, crumbly edge, gap under the nail |
| Drug or pigment staining | Dark banding or diffuse color shift | Starts after a new medicine or repeated nail product use |
Clues That Help Tell A Bruise From Something Else
A bruise under the nail usually follows a story you can point to. You banged your toe. You ran a race. Your shoes felt tight. The spot may hurt early on, then settle. Over the next few months, the dark area should move forward as the nail grows.
A dark mark that stays rooted in the same spot is different. So is a new vertical band, uneven color, or color that reaches onto the skin beside the nail. The American Academy of Dermatology’s nail melanoma signs page shows the kind of changes skin doctors want checked. Mayo Clinic also notes that melanoma under a nail can appear dark, flat, and uneven in shape on its melanoma symptoms and causes page.
If the nail is thick, crumbly, or lifting, a fungal cause rises on the list. The NHS nail problems guide also points out that nail color changes can come with thickening or overgrowth, which helps separate a slow-moving nail problem from a fresh bruise.
When The Nail Falls Off
That can happen after a hard bruise. It looks dramatic, but the old nail may loosen as a new nail grows in from the base. The area can stay tender and odd-looking for a while. Toenails grow slowly, so full replacement often takes many months.
Keep the area clean, dry, and protected from more rubbing. If there is pus, spreading redness, fever, or rising pain, get it checked.
| What You Notice | What It Often Means | When To Get Checked Soon |
|---|---|---|
| Dark spot after a stubbed toe | Bruise under the nail | If pain is severe, the nail is split, or walking is hard |
| Dark area that grows out with the nail | Old blood from trauma | If it stops moving or starts spreading |
| Black or brown vertical band | Pigment change or, at times, melanoma | If it is new, widens, or reaches nearby skin |
| Thick, brittle, dark nail | Fungal infection is common | If it spreads, hurts, or the skin gets inflamed |
| Loose nail with odor or drainage | Infection or debris under a lifted nail | Get checked promptly |
When A Black Toenail Needs Medical Care
Some black toenails can wait and be watched. Others should be seen sooner. The biggest red flags are change, spread, and lack of a clear injury story.
- You did not injure the toe and the dark mark appeared anyway.
- The spot does not move outward as the nail grows.
- A dark streak is getting wider or darker.
- Color spreads onto the skin at the cuticle or nail folds.
- The nail is splitting, lifting, or changing shape.
- There is drainage, redness, heat, or bad odor.
- The toe hurts enough to limit walking.
- You have diabetes, poor circulation, or reduced feeling in the feet.
A clinician may inspect the nail with magnification, clip part of the nail for fungus testing, drain trapped blood if the injury is fresh and painful, or refer you for a biopsy if the pigment looks suspicious.
What You Can Do At Home Right Now
If the black nail followed a clear bump or shoe pressure and there are no red flags, basic care is usually enough. Wear roomy shoes. Keep the toe dry. Avoid picking at the nail. Trim only the loose edge if it catches. Don’t force off a nail that is still attached at the base.
Take a photo every few weeks. That makes it easier to tell if the mark is moving forward with growth. If the spot stays fixed or changes in a bad direction, book an appointment.
One last point: a black toenail can be harmless, but “harmless” should reveal itself with time. A bruise grows out. A friction injury settles once the pressure stops. A mark that breaks those rules deserves a closer check.
References & Sources
- American Academy of Dermatology.“How to Check Your Nails for Melanoma.”Shows warning signs of melanoma under or around a nail, including dark streaks and pigment spread onto nearby skin.
- Mayo Clinic.“Melanoma – Symptoms and Causes.”Notes that melanoma can occur under a toenail or fingernail and may appear dark, flat, and uneven.
- NHS.“Nail Problems.”Lists nail color changes, thickening, and other patterns that help place black toenails in a broader nail-health context.