The black color of the menstrual precursor may be caused by poor discharge of menstrual blood, endocrine disorders or endometritis. It is recommended to consult a doctor in time to find out the cause and treat it under the doctor’s guidance. 1. Poor discharge of menstrual blood: If the amount of menstrual blood is not very large when you first have your period, and the flow of menstrual blood is slow and not easy to discharge, it is easy to stagnate in the body. As there is iron hemoglobin in menstrual blood, if it cannot be discharged for a long time, it may be oxidized, which will lead to the black color of menstrual precursor. 2. Endocrine disorders: If women experience a decrease in estrogen secretion, it may lead to the emergence of a thin uterine lining and low menstrual flow, so that the menstrual blood can not be discharged from the uterine cavity in a timely manner, which may lead to the emergence of black menstrual blood. 3. Endometritis: If a woman suffers from endometritis, due to the influence of pathogenic microorganisms, it may lead to abnormal menstruation, which may result in black menstrual blood. If the bleeding is black for a long period of time, it is recommended to go to the hospital in time for consultation and standardized treatment after clarifying the cause of the disease.