When my gf was young her brother dropped a book on her toes and nw its black under the nail. How do you get rid of the blackness/purpleness?

Have it evaluated. It is not unusual to have the nail look black underneath after trauma. Many times the nail will fall off and a new one will grow out uneventfully. It should not stay black. Have it checked out. Dr l.
Still black. If they are still black from childhood she needs to be checked.