A light black line on the thumbnail may be caused by a nevus of onychomycosis, localized trauma, systemic pigmentary diseases, or certain fungal infections, eczema-like changes, etc. The nevus of onychomycosis is a kind of nevus that grows at the root of the thumbnail. 1. Nail nevus: Nail nevus is a kind of nevus that grows at the root of the nail, which will grow with the nail and cause a light black line formed by melanin on the surface of the nail, and usually there is no obvious itching or pain. 2. Local trauma: If the thumbnail area is squeezed or collided, it will lead to obvious blackening of the nail, accompanied by irregular changes in the shape, and there may be a light black line on the top of the nail. 3. Systemic pigmentary diseases: If you suffer from systemic pigmentary diseases, it may cause melanin activation and proliferation, which may cause a light black line on the thumbnail. A light black line on the thumbnail may also be other reasons, it is recommended to go to the hospital to do a checkup, clear the cause of timely treatment.