Suddenly black toenails may be caused by onychomatous nevus, subnail bruise, melanoma and so on. 1. Mole of the nail: it is a benign tumor that grows on the nail area, due to the abnormal proliferation of melanocytes, so it may cause toenails to turn black suddenly. 2. subnail bruise: for example, women often wear high heels, or the toenail is subjected to external extrusion, smashing and so on, causing subcutaneous capillary rupture, resulting in subnail bruise, which can also cause toenail suddenly turn black. 3. Melanoma: it is a malignant tumor originated from melanocytes, which may also cause the appearance of toenails turning black suddenly, and it needs to take measures such as surgery, radiotherapy and chemotherapy as soon as possible. Toenails suddenly turn black, there may be other reasons, it is recommended to go to the hospital for examination, to clarify the cause of the disease, and follow the doctor’s instructions for treatment.