Black-and-Blue Nails

A black-and-blue nail is usually caused by sudden or repetitive injury to a toe. This might occur during sports that involve running or stopping quickly. The injury may also result from a heavy object falling on a toe. If your toe is black and blue but not injured, see your doctor immediately.

Symptoms
The big toe is the most often affected. Bruised, broken blood vessels cause the black-and-blue colors under the nail. If the condition is the result of a sudden injury, pain may be severe.