What causes black nail discoloration?
Black nail discoloration can happen for simple reasons, such as minor trauma, or for more serious medical conditions that need prompt attention.
Understanding the pattern, timing, and associated symptoms can help you judge whether it is likely to resolve on its own or needs evaluation.
The color change may affect a single nail or several nails, appear under the nail plate, start at the cuticle, or involve the skin around the nail.
In many cases, the cause is visible once you look closely at recent injuries, footwear, medications, and changes in skin or health.
Common causes of black nail discoloration
1. Subungual hematoma from injury
The most common cause of a dark or black nail is a subungual hematoma, which is bleeding under the nail after trauma.
This can happen from dropping something on the toe, stubbing a finger, running long distances, or wearing tight shoes that repeatedly compress the nail.
Blood trapped beneath the nail often looks red, purple, brown, or black as it clots.
Pain is common at first, especially if pressure builds under the nail, and the discoloration usually moves outward as the nail grows.
2. Repetitive pressure and friction
Repeated stress on the nail can create bleeding or dark staining without a single dramatic injury.
Runners, dancers, hikers, and people who spend long hours in shoes with a narrow toe box are especially prone to this problem.
Toenails are affected more often than fingernails because of ongoing impact during walking and exercise.
If the discoloration follows a pattern of repeated shoe pressure, the cause is often mechanical rather than infectious or malignant.
3. Fungal nail infection
Onychomycosis can darken nails, although fungal infections more often cause yellow, white, or brown changes before the nail becomes thick, brittle, and distorted.
In some cases, debris under the nail and secondary changes in the nail plate make the nail appear very dark.
Fungal infection is more likely when multiple nails are affected, the nail is thickened or crumbly, and the changes develop slowly.
A clinician may confirm the diagnosis with microscopy, culture, or other testing before recommending treatment.
4. Pigmentation from medications or chemicals
Some medications and topical exposures can discolor the nails.
Drugs reported to cause nail pigmentation include certain antibiotics, antimalarials, chemotherapy agents, and medications that affect melanin production or deposition.
Hair dyes, silver-containing products, tobacco staining, and occupational chemical exposure may also contribute.
In these cases, the discoloration may involve multiple nails and may be accompanied by staining of the surrounding skin.
5. Pigment from the nail matrix
Dark streaks or patches can form when pigment is produced by the nail matrix, the tissue that makes the nail plate.
This may be benign, as in melanonychia, or concerning if the pigmentation is changing rapidly or affecting only one nail.
Benign melanonychia is more common in people with darker skin tones and can appear as a stable brown or black band.
New, irregular, widening, or uneven pigmentation should be examined carefully.
When black nail discoloration may be serious
Could it be acral lentiginous melanoma?
One of the most important diagnoses to consider is acral lentiginous melanoma, a type of skin cancer that can develop under the nail or on the nail unit.
It may present as a dark band, a patch of pigment, or a lesion that darkens and enlarges over time.
Warning signs include a single nail affected without clear trauma, pigment that extends onto the surrounding skin, irregular borders, widening of the band, nail splitting, or bleeding.
This cancer is less common than injury-related discoloration, but early detection matters.
What about infection or systemic disease?
Black or very dark nails can occasionally reflect underlying illness, including vascular problems, endocrine disease, or inflammatory conditions.
Melanoma, bacterial infection, and severe fungal disease can also alter nail color and shape.
If the nail looks black with redness, warmth, swelling, pus, fever, or severe pain, infection should be considered.
If multiple nails change color along with fatigue, bruising, or other body-wide symptoms, a broader medical evaluation may be needed.
How clinicians evaluate black nail discoloration
Evaluation usually starts with a history of recent trauma, running, ill-fitting shoes, manicure or pedicure injury, new medications, and changes in skin or nail growth.
A clinician will also look at whether the color is under the nail, within the nail plate, or on the surface.
- Physical exam: checks the shape, color, thickness, tenderness, and surrounding skin.
- Dermoscopy: helps distinguish pigment from blood and identify suspicious patterns.
- Fungal testing: may include scraping, microscopy, or culture if infection is suspected.
- Biopsy: may be recommended if melanoma or another tumor cannot be ruled out.
For an obvious subungual hematoma, treatment may simply involve observation if the nail is stable and pain is manageable.
In some cases, drainage is performed soon after injury to relieve pressure, but this is time-sensitive and should be done by a clinician when appropriate.
What black nail discoloration looks like by cause
Blood under the nail
Traumatic bleeding usually looks like a dark red, purple, or black area that matches the shape of the injury.
It often causes throbbing pain early on and then gradually becomes less painful as pressure decreases.
Fungal changes
Fungal discoloration often comes with thickening, crumbly edges, lifting of the nail, and a slower progression.
It may start as yellowish or brown and become darker as debris accumulates.
Pigment band from melanonychia
Melanonychia usually appears as a vertical band that runs from the cuticle to the tip.
Stable bands, especially in several nails, are often benign, while a single changing band deserves prompt assessment.
Suspicious lesion under the nail
A concerning lesion may look like a dark spot or streak that does not grow out normally, becomes irregular, or spreads to the skin beside the nail.
Any unexplained change that persists should be taken seriously.
What you should not do at home
Avoid drilling, cutting, or trying to drain a black nail on your own.
Home procedures can increase infection risk, worsen injury, and delay the diagnosis of serious causes.
Do not assume all black nail discoloration is a bruise, especially if there was no trauma or if the color is changing, recurring, or limited to one nail.
If polish or artificial nails are present, remove them carefully so the nail can be assessed properly.
When to seek medical care
Seek medical evaluation if the discoloration is unexplained, painful, growing, or associated with nail distortion.
Prompt care is especially important if the pigment is new on one nail, extends onto the surrounding skin, or is not gradually moving outward with nail growth.
- Severe pain after injury
- Black discoloration without a clear cause
- One nail with a new dark band or patch
- Bleeding, pus, warmth, or swelling
- Nail splitting, lifting, or deformity
- Discoloration that does not improve over time
If a child, older adult, or person with diabetes, poor circulation, or immune suppression has nail discoloration, medical review is even more important because complications can develop faster.
How to reduce the risk of black nail discoloration
Prevention depends on the cause.
Protective footwear, proper nail trimming, and attention to athletic training can lower the risk of trauma-related bleeding under the nail.
- Wear shoes with enough toe room.
- Trim nails straight across and avoid cutting them too short.
- Use gloves or protective gear when appropriate.
- Change wet shoes and socks quickly after exercise.
- Review new medications if multiple nails darken after starting treatment.
- Monitor any persistent nail streaks or spots for change.
Because black nail discoloration has a wide range of causes, the key is pattern recognition: sudden pain after injury points one way, while a persistent unexplained pigment change points another.
Paying attention to those details helps separate temporary nail damage from conditions that need diagnosis and treatment.