There are various reasons such as non-pathological and cancerous changes in moles becoming black. 1. Non-pathological: when there is more sun exposure, the nevus will become darker and bigger accordingly; during pregnancy, puberty or taking corticosteroid or contraceptive pills, the nevus may also become darker; non-pathological nevus becoming darker usually does not need special treatment. 2. Cancer: If the color change of nevus occurs within several months, such as blackening, or its size, shape and elevation change, or itching, pain, bleeding and other symptoms appear, or satellite foci appear around the original nevus, it cannot be excluded that the nevus may be cancerous, and it should be standardized and diagnosed as early as possible. There may be other reasons for moles to become black, it is recommended to go to regular hospitals in time, under the guidance of specialists to carry out targeted treatment, in order to delay the condition.