Black toenails (subungual hematomas) are bruised toenails that result from blood collecting under the nail. They can be caused by crushing, impact, or injury to the toe, such as dropping something heavy on or stubbing the toe. Runners frequently suffer from black toenails due to the big toe rubbing against the tip of an ill-fitting sneaker. Although black toenails can be painful and tender, treatment is usually not necessary unless there is bone or tissue damage, too.