Black nails often come from injury, pigment changes, fungus, or, in rare cases, melanoma that needs prompt medical care.
A black fingernail or toenail can be startling, but it does not point to one single cause. In many cases, the color is old blood trapped under the nail after a hit, a stubbed toe, or repeated rubbing from tight shoes. That kind of bruise is common, and the color often shifts as the nail grows.
Still, not every dark nail is a bruise. A new black streak, a patch that starts near the cuticle, or color that spreads onto the skin beside the nail can point to something that needs a proper skin check. The pattern matters. So does whether one nail changed or several did.
This article breaks down the usual causes, the warning signs that deserve faster care, and what you can check before you book a visit.
Why Are My Nails Turning Black? Common Triggers By Nail Pattern
The first clue is the way the color looks. A solid dark patch after trauma tells a different story from a thin dark band that grows wider over time. Pain, swelling, nail thickening, and the number of nails involved also change the list of likely causes.
One nail that turns black after a clear bump or crush often means a subungual hematoma. That is blood under the nail. One nail with a new dark streak and no injury raises more concern. Several nails changing at once leans more toward pigment, staining, medicine effects, or a skin condition.
The Most Common Cause: Dried Blood Under The Nail
If you dropped something on a finger, jammed a toe, or spent days in snug shoes, blood can collect under the nail and turn it dark red, purple, brown, or black. The spot may ache or throb at first. Later, the pain eases, but the color hangs around while the nail grows out.
Toenails often bruise from repeat friction. Runners, hikers, and people who wear short toe boxes see this a lot. A bruise usually grows forward with the nail. If the dark area stays fixed near the base, or you never had trauma, that is less reassuring.
Black Streaks Need More Attention Than Many People Expect
A narrow brown-black band can come from harmless pigment, especially if it has been stable for a long time or appears on more than one nail. But a new streak on one nail, a streak that widens, or a stripe with uneven borders needs a closer look.
The American Academy of Dermatology’s nail melanoma guidance warns that melanoma can show up under or around a nail. It is rare, yet this is one cause you do not want to wave off as “just a bruise.”
| What You See | Likely Cause | What To Do Next |
|---|---|---|
| Black or purple patch after a hit | Bruised nail or trapped blood | Watch it grow out; get checked if pain is strong or the nail is split |
| Dark spot on one toenail after long runs or tight shoes | Repeated friction | Change footwear and track whether the spot moves forward with growth |
| Thin brown-black band that has stayed the same for years | Stable pigment band | Book a routine skin or nail visit if you are not sure what it is |
| New dark streak on one nail that gets wider | Pigment growth from the nail matrix | Arrange a skin check soon |
| Dark color plus thick, crumbly, misshapen nail | Fungal nail infection | Get it confirmed before treatment |
| Dark patch with swelling, pus, or sore skin | Infection or trauma | Book prompt care |
| Color on several nails at once | Pigment, staining, medicine effect, or skin disease | Review new products, medicines, and skin symptoms with a clinician |
| Dark color that reaches the nearby skin | Melanoma warning sign | Seek urgent assessment |
When Fungus, Pigment, Or Friction Is More Likely
Fungal nail infection does not always look black. More often, the nail turns yellow, white, or brown and becomes thick, rough, or crumbly. Still, some nails look so dark that people call them black, especially when debris builds under the nail or the plate starts to lift.
The NHS page on fungal nail infection notes that fungal nails tend to become discoloured, thickened, and easy to break. That texture clue helps separate fungus from a simple bruise. A bruise changes color and grows out. Fungus often lingers, spreads, and changes the nail’s shape.
Pigment can also darken nails without injury. Some people get harmless brown-black bands from melanin. These bands can stay stable for years. They are more often seen on several nails than melanoma is. Staining from dark polish, hair dye, nicotine, or other products can also mimic a black nail, though that color usually sits on or near the surface rather than deep under the nail plate.
If your black nail showed up after a season of running, football, hiking, or work boots, friction climbs high on the list. The nail may not hurt much by the time you notice it. Many “black toenails” in active people start this way.
The MedlinePlus page on nail injuries lists bleeding under the nail as one of the usual signs after nail trauma. That fits the dark, bruised look many people see after a hit or crush.
What You Can Check At Home Before You Book A Visit
- Did one nail change, or several?
- Was there a clear injury, long walk, race, or tight-shoe day before it showed up?
- Is the dark area a patch, a thin streak, or a broad band?
- Is there pain, swelling, drainage, or a bad smell?
- Is the nail getting thick, brittle, or lifted?
- As the nail grows, does the dark area move forward or stay anchored near the base?
- Has any color spread onto the skin beside the nail?
Those details can shorten the guessing when you see a GP, dermatologist, urgent care clinician, or podiatrist.
Signs That Need Faster Medical Care
Some nail changes can wait a bit. Others should not. Severe pain after a fresh crush may mean trapped blood with pressure under the nail or a fracture in the fingertip or toe. A new black streak with no injury deserves prompt review, even if it does not hurt.
| Red Flag | Why It Matters | How Soon To Get Care |
|---|---|---|
| New dark streak on one nail | Needs a melanoma check | Book soon |
| Band gets wider or more uneven | Growth pattern is more concerning | Book soon |
| Color reaches the skin near the nail | Classic warning sign for nail melanoma | Urgent review |
| Severe throbbing after a crush injury | Pressure under the nail or a broken bone | Same day or next day |
| Pus, marked swelling, fever, or spreading redness | May point to infection | Prompt care |
| Nail splits, lifts, or bleeds with no clear injury | Needs a proper nail-bed check | Book soon |
What A Clinician May Do
The visit is often straightforward. You will be asked when the color started, whether there was trauma, what medicines you take, and whether the spot has changed. The nail and nearby skin will be checked closely. A bruise is often diagnosed from the story and appearance alone.
If fungus is on the list, the nail may be clipped or scraped for testing. If melanoma is a concern, you may need a nail unit biopsy. That sounds daunting, yet it is the right step when a streak or patch does not fit a harmless pattern.
Care That Usually Helps While The Nail Grows Out
If the dark nail came from a mild injury and the pain has settled, the main job is to protect the nail while it grows. Keep it trimmed, avoid more trauma, and wear shoes with enough toe room. Do not try to drill or cut the nail at home.
If the nail is thick, crumbly, or lifting, treatment depends on the cause. Fungal nails often need confirmation before pills or long courses of topical treatment make sense. If the nail is partly detached, keeping it clean and dry lowers the chance of irritation and infection.
A black nail that slowly moves outward is often old blood doing what old blood does. A dark streak that stays rooted near the cuticle is a different pattern. That is the sort of change worth getting seen, even if it feels easy to put off.
When The Color Does Not Grow Out
This is the part many people miss. Bruised nails move. They may take months to clear, more so on toes, but they usually march forward with the nail. Color that stays put, keeps widening, returns in the same place, or spills onto the nearby skin should not be treated as a waiting game.
Most black nails are not cancer. Many are simple bruises. Still, a new or changing dark streak is one of those signs that earns a closer look. If you are torn between “watch it” and “get it checked,” the safer call is clear when there was no injury, the nail pattern looks odd, or the color is not growing out.
References & Sources
- American Academy of Dermatology.“How To Check Your Nails For Melanoma.”Lists warning signs of nail melanoma, such as new or changing dark streaks and pigment that reaches nearby skin.
- NHS.“Fungal Nail Infection.”Describes the usual look of fungal nails, such as discolouration, thickening, and easy breakage.
- MedlinePlus.“Nail Injuries.”Explains trauma-related nail changes, such as bleeding under the nail after a crush or impact.