It is a normal physiological phenomenon to have black menstrual blood in the late menstrual period and in relatively small amounts. Normal menstruation lasts for about 5 days and the blood flow has decreased by the late menstrual period. The percentage of vaginal secretions is higher, and the vaginal secretions and menstrual blood mix together to turn black. If the color of menstrual blood is black and the amount is low during the whole period, it is abnormal. It may be caused by cold in the abdomen or eating cold food during the period. It may also be caused by gynecological inflammation, such as endometritis, endocrine disorders, etc., which can also lead to black menstrual blood and low volume.