Blue nevus. A blue nevis is a very dark mole. This can not be determined just by appearance. Since moles can be abnormal a very dark mole should be evaluated and possibly biopsied by a dermatologist. Even benign lesions can change and a delay in diagnosis and treatment of a melanoma can make a significant difference in prognosis.
Abnormal. A black mole is abnormal, particularly if some parts are darker than others, it has irregular borders, or if it is larger than a pencil eraser. See a dermatologist about having it removed due to the risk of melanoma. Your dermatologist can tell you if it is likely to be benign.