A piece of blackened toenail cover may be related to bruising under the toenail, fungal infection, pigmented nevus and other reasons. 1. Bruising under the toenail: If the toenail is smashed, it can cause the capillaries under the toenail to rupture, resulting in bruising, which is manifested as a black nail cover. 2. Fungal infection: when the toenail is infected with fungus, it is easy to form black pigment, which makes the black block appear in the local toenail. 3. Pigmented nevus: When pigmented nevus appears under the toenail cover, it may appear as a blackened area under the toenail cover. It is suggested that a black toenail cover is also seen in other causes, it is recommended to go to the regular hospital in time, to clarify the cause of the disease, under the guidance of the doctor for standardized treatment, so as not to delay the condition of the body to bring adverse effects.