A black, purple or brownish discoloration under or involving a toenail is frequently due to trauma to the toenail, 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.
When the second or third toenails are involved, it is commonly referred to as "runner's toe." This can be the result of the nail being slightly too long and the shoe being either too big or too tight. If the shoe is too big, when running down hill, the foot slips and the nail can get caught where the toe cap meets the toebox. If the shoes are too tight, the nail can get pinched and jammed, resulting in bleeding between the nail plate and nail bed.
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.