What causes black and blue toenails?

Trauma: Usually trauma or injury will cause bleeding under the nail and the appearance of being black & blue.

See a podiatrist: If you are runner, this is a common injury called "black toe syndrome" due to blood bruises under the nail plate. Other causes of black toenails are infections - either bacterial or fungal. Another cause may be from a fracture in the last bone of the toe. In any case, see a podiatrist who can examine and treat any of these conditions and recommend ways to prevent recurrence.