Thumb nail black, consider and subnail bruise, nail mother mole, subnail melanoma and other factors caused. 1. Subnail bruise: When the nail is subjected to external force, such as smashed or crushed by heavy objects, subnail bleeding may occur. If bruises are formed, it may lead to blackening of nails. 2. Onychomycotic nevus: It is found in children’s thumb nails and often manifests as brown pigmented bands of uniform color. It may also cause black nails. 3. Subungual melanoma: some patients with melanocytoma may show uneven color of black-brown pigmented bands on the nails in the early stage, and then the disease progresses rapidly, resulting in nail breakage, bleeding and other symptoms. There may be other causes of black thumb nails, it is recommended to go to the hospital in time, clear diagnosis, symptomatic treatment.