A black, purple, or brownish discoloration under or involving a toenail is frequently due to trauma to the toe nail, such as when something is dropped on the toe. The color results from a blood clot or bleeding under the nail, and may involve the entire nail or just a small portion of it. This can be very painful when the entire nail is involved, and may need medical attention to relieve the pressure caused by bleeding under the toenail.
Although it is very rare, a more serious cause of black toenails is malignant melanoma. Since early diagnosis and treatment of melanoma improves the chances for a good outcome, it is important that all black toenails be evaluated by a qualified foot and ankle surgeon to rule out this cause.
Other rare causes of black toenails include fungal infections, chronic ingrown nails, or health problems affecting the rest of the body.
Courtesy of Foot Health Facts www.foothealthfacts.org