What should i do with a black toenail?

Black Toenail : If it was caused by trauma bleeding under the nail leave the nail in place and allow a new nail to develop. I would have a podiatrist take a look at it if it’s painful or doesn’t clear as the new nail grows.

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.

Black toenail: Have it examined, & treat the cause, such as trauma.

Black toenail: If there was any type of injury it's likely a subungual hematoma but could be a number of things so play it safe and get checked.