The difference between a nevus of onychomycosis and a nail black line is that a nail black line is a symptom and a nevus of onychomycosis is a disease. A nevus of onychomycosis can manifest as a nail black line, but a nail black line can be caused by a variety of factors. A nail nevus is a pigmented cell nevus that grows on the nail plate and is a benign skin tumor. As the nail plate grows, well-defined black bands can be seen on the nail plate. Nail black line is a symptom that refers to the black band-like changes on the nails caused by various reasons. Onychomycosis is the most common cause of onychomycosis. However, there are other causes, such as exposure to dyes, drug deposits, and melanoma occurring at the root of the nail. Therefore, there is a fundamental difference between a black line on the nail, which is a symptom, and a nevus, which is a disease. If you have black nail lines or suspect that you have a nevus of the nail, you can go to the dermatology department of the hospital, under the guidance of the doctor to clarify the cause of the disease, and then treat the symptoms in a timely manner.