There are many reasons for black nails, such as malignant melanoma of nails, subnail hemorrhage and so on. 1. Malignant melanoma of nails: malignant melanoma of nails has a higher degree of malignancy, which can show black nails, especially in the nail bed and nail plate position, which can show a band of pigmented stripes, and even cause lymph node metastasis and ulcers. 2. Bleeding under the nail: external extrusion, collision and other factors will cause bleeding under the nail, if the bruising is more serious, it can lead to the phenomenon of black nails, usually after the absorption of bruising will be relieved. The causes of black nails are many, need to go to the hospital in time to improve the examination, it is recommended that patients with symptoms in a timely manner, under the guidance of the doctor to take treatment, avoid self-medication.