The blackening of thumb toenail may be related to trauma, onychomatous nevus, onychomelanoma and other diseases. 1. Trauma: after the toenail is traumatized, the blood vessels and nerves under the nail may be damaged. When the blood vessels are ruptured and the blood is not discharged from the body, bruises will be formed under the nail, which will be manifested as the blackening of the toenail in this case. 2. Nail nevus: this disease is a kind of benign tumor with pigmented nevus cells in the nail nevus, the cause of which has not been clarified yet, and it mainly manifests as longitudinal black lines on the nail plate of finger/toe nails in the clinic and usually has no self-conscious symptom. 3. Nail melanoma: Nail melanoma is a kind of malignant tumor, which can be transformed from nevus of nails, and in clinic, it can be manifested as a large area of diffuse nail discoloration under the nail plate, which can be accompanied by thickening and destruction of nail plate, and even erosion, ulceration, bleeding and so on. It is recommended that patients with black toenails go to the dermatology department of regular hospitals for a clear diagnosis and choose the appropriate treatment plan under the guidance of doctors.