A bruised nail is as a result of accidental trauma. Trauma over the nail bed can result in a bruise that appears as a very dark area under the nail plate where blood is collecting. It is possible that this can cause a lot of pressure and pain and a visit to a medical practitioner is needed to release the pressure. The discolouration will grow out with the nail. If trauma happens over the matrix which is under the base the nail, it can result in the new nail that is forming there to become detached which will eventually grow and be lost as the new, undamaged nail, pushes it off. As the departing nail is growing out, care should be taken not to catch the broken edge of the nail plate and pull it off.