The blackening of nail edges may be related to trauma, onychomycosis, onychomatous nevus and other diseases. 1. Trauma: If the nails are squeezed or hit by the outside world, it may lead to localized bruising, which will result in blackening of the nail edges. 2. Ringworm of the nails: the cause is mostly related to the infection of skin ringworm bacteria, and local blood circulation disorder and impaired immunity may also induce the disease. The nails of patients will be separated, discolored and turbid, and the edges of nails of some patients will be blackened. 3. Nail nevus: a pigmented nevus that appears on the nail. Black or gray longitudinal lines will appear on the nails, which may lead to blackening of nail edges. There are many reasons for the blackening of nail edges, so it is recommended to go to regular hospitals for examination and clarification of the cause of the disease.