The blackening of the thumbnail may be caused by trauma, onychomycosis, and onychomycotic nevus, among other factors. 1. trauma: blackened thumb nails may be caused by trauma, if directly suffered external injury, may lead to bleeding bruises under the nail, thus triggering local blackening. 2. ringworm of the nail: ringworm of the nail patients will appear discoloration, thickening and separation of the situation, some patients may appear black thumb symptoms. The cause is fungal infection of nails. 3. Onychomycosis: the nails of patients with onychomycosis will appear nail black line, and its specific etiology is still unclear. There are many reasons for the blackening of the thumb nail, and it is recommended that patients go to regular hospitals for standardized diagnosis and treatment by professional doctors.