Black toenails may be related to nail trauma, or may be due to signs of diabetic foot, onychomycosis and other diseases. 1. Nail trauma: In the course of ordinary work or life, if the toenail is accidentally extruded or smashed by external forces, resulting in bleeding under the nail, it may cause the toenail to become black. It may also be accompanied by localized redness, swelling and pain in the toes. 2. Diabetic foot: this disease is a complication of diabetes, mainly due to vascular and lower extremity neuropathy caused by foot infections, which can lead to black toenails, but also may be accompanied by localized black toes, intermittent claudication and other symptoms. 3. Nail nevus: If the toenail is black, mainly manifested as line-like blackness, it is considered that it may be caused by nail nevus. It has many triggers, which may be related to subnail hemorrhage, eczema-like changes, fungal infection and other reasons. Black toenails may also be caused by other reasons, if accompanied by obvious symptoms of discomfort, it is recommended to consult a doctor as soon as possible, after diagnosis of the cause of the disease, to give targeted treatment and conditioning.