The most common culprit for black nails is repetitive trauma, which can result from running or from wearing any type of ill-fitting footwear. If a black nail crops up shortly after a workout or a day spent in too-tight or too-loose shoes, this is likely the cause.

Repetitive trauma ranges from mild (think: a small, painless, black-and-blue discoloration beneath the nail), to severe (large, bloody blisters between your nail and nail plate). In mild cases, no treatment is needed, and the black nail will simply grow out.

In severe cases, beneath-the-nail blisters can cause the nail to detach—either partially or fully—from the nail plate. This process can be quite painful if the detachment is only partial.  Once the nail fully separates from nail plate, it is officially dead and will never reattach. The good news is that it’s no longer painful. The bad news? It can take a long time for a new nail to grow in—about a year for big toenails and three to six months for smaller nails. In certain cases, a fresh nail can begin growing underneath an old, dead nail.

If there’s additional repetitive trauma, the new nail can become bruised and detached as well. To prevent this, you can trim down or entirely remove the dead nail, which will allow the new nail room to grow in properly.

Another time you should visit your doctor is if the skin surrounding your blackened nail is red, inflamed, or oozing. This may be a sign of an infection and you should apply an antibiotic ointment until you can get an appointment.

To avoid black toenails caused by repetitive trauma, either trying a bigger shoe or wearing a thinner sock (thick socks may cause too much pressure on your toenails).

Dropping a heavy object (say, a dumbbell) onto your foot can burst the blood vessels under your nail bed and cause blood to pool underneath. This type of black nail—clinically called subungual hematoma—is especially easy to identify, as it will appear almost immediately after an incident.

The build-up of blood typically causes a painful throbbing sensation that can be addressed by pricking a tiny needle through the nail to drain out the blood. This procedure will alleviate both the pressure and dark color under the nail—and should always be done by your doctor, Sutera says. At-home attempts are often unsanitary, ineffective, and more excruciating than in-office care.

source: runners world