The black toenail looks like old nail polish. You can see a white band under it indicating fresh toenail growth.
If you stub you toe hard enough to kill the toenail, it might turn black and grow back translucent like healthy nails.
Here are six causes of black toenails.
1. Repetitive trauma
When a person wears poorly-fitting shoes, they may be at risk of developing black toenails due to repetitive trauma.
Long-term pressure on the toes from poorly-fitting shoes can cause a range of problems, from small blisters to bloody blisters under the nail.
In mild cases, the black toenail will grow out naturally over time without treatment. In severe cases, such as when the nail begins to detach from the nail bed, a person should seek medical treatment.
2. Blunt force
In some cases, the trauma may be a one-time blunt-force injury, for example, if a person drops a heavy object on their foot or toes. When this happens, blood vessels in the nail bed break, causing blood to pool there.
The injured toe will start to hurt and pool blood beneath the skin almost immediately. The buildup of blood will cause the toe to feel painful and swollen. A doctor can treat this condition by draining the blood with a pinprick.
3. Fungal infections
Fungal infections are another common problem that can cause black toenails to form. Typically, a fungal infection causes a white or yellowish discoloration. However, debris can build up near the infection, causing the nail to appear black.
Toenails are particularly susceptible to fungal infections, as socks and shoes can provide a warm and moist breeding ground for a fungus to become established. Fungal infections can usually be prevented with proper foot care.
4. Melanoma
In rare cases, a black toenail may be caused by melanoma. Melanoma is the most aggressive form of skin cancer.
Melanoma causes the skin to develop one or more patches of dark, irregular-looking skin. In some cases, the patch of darkening skin grows beneath the nail bed.
Melanoma develops slowly, and without other symptoms, so it is often not noticed in the early stages, especially if it originates under the toenail.
5. Pigmentation changes
People's skin tones can naturally change over time. For people with darker skin, a dark patch may develop beneath the toenails.
Pigmentation changes are usually uniform, so if it affects a toe on one foot, it will usually affect the matching toe on the other foot. The fingernails may develop darker patches beneath them as well.
6. Underlying conditions
There are a few medical conditions that may cause black toenails, including:
diabetes
kidney disease
heart disease
anemia
In most cases, controlling the underlying condition will help the nail to regain its original color.
