What are black marks under fingernails?

Did you bruise your finger in any way? Usually, blood from damaged blood vessels deep beneath the skin collects near the surface of the skin resulting in what we think of as a black and blue mark. Like the bruise you get after getting your finger caught in a door.

A fresh bruise may actually be reddish. It will then turn blue or dark purple within a few hours. If it's still painful after a couple of days you may want to consult a medical professional

The disease can be Melanonychia caused due to increased production of melanin by melanocytes in the nail matrix.