When patients find a black toenail, they can first determine whether they may be due to trauma, whether there is a family history of melanoma and may also exist fungal infection, it is recommended that under the guidance of a doctor after a clear diagnosis for symptomatic treatment: a. under the nail bleeding: toenail bleeding and no oxygen, resulting in blood stagnation seems black, visible to the naked eye toe nail black. 1, bruising: because of trauma or long-term running, etc. It is a bruise under the nail due to trauma or prolonged running. The border is clear and can be irregular, and can heal on its own after a period of time. 2. Bleeding diseases: Rarely seen, bleeding under the nail may be caused by diseases with a tendency to bleed, such as fulminant purple scar, subacute bacterial endocarditis and hematological diseases. 2. Nail fungus: local cracks, brittle or thickened, brown or black, may form after the appearance of nail infection, showing as black toenails. It is often caused by fungal infections, and nail fungus can also be triggered by living in an overly humid and warm environment and poor personal hygiene. In addition, onychomycosis is contagious and can be transmitted directly or indirectly to others, so it can also be caused by infection from people with onychomycosis. Nail nevus: The most common symptom of nail nevus is a black line, which may make the nail completely black. There is a small chance of nail nevus becoming cancerous, so you need to go to the hospital to identify it. 4. Limb freckle-like melanoma: It is seen as a result of genetic factors or ultraviolet radiation, which induces melanocytes to turn into atypical melanocytes or melanoma. It is common in limb areas such as under the nail, occurs in the nail bed, is brown or black, and may show up as a black toenail.