It may be response to irritation from a fungual or bacterial infection. Rarely, it may be a sign of the skin cancer melanoma. See doctor.
Here are some of the signs of subungual melanoma:
-Spread of pigmentation into the nail folds.
-Pigmentation in a single digit
- Occurs at age 50 or older
- Occurs in the thumb, index finger, or great toe
- Blurred borders
- History of melanoma