A black line on the back of the calf may be caused by pigmentation or varicose veins. 1. Pigmentation: Common inflammatory skin diseases, autoimmune diseases, or light factors may cause abnormal pigment metabolism, resulting in pigmentation, so there may be a black line behind the calf. 2. Varicose veins: the veins adjacent to the surface of the skin are tortuous and dilated, most commonly in the legs and ankles, manifested as dark blue, swollen and twisted subcutaneous veins, which may also be manifested as a black line behind the calf. There are many other causes of a black line at the back of the calf, it is recommended to go to the hospital in time, under the guidance of the doctor to clarify the diagnosis and give standardized treatment.