The reasons for black blood flowing from the vagina are as follows: a. If black blood flows during menstruation, most of it is due to the accumulation of blood in the uterine cavity staying for a long time and not discharged in time. If you eat more warm food or take medicine to promote pelvic blood circulation such as motherwort granules, Gui Zhi Fu Ling capsule and Wu Ji Bai Feng Wan, the blood will turn from black to dark red. Second, black blood from the vagina occurs during the non-menstrual period, which may be related to the presence of submucosal fibroid necrosis, endometrial polyp necrosis or cervical inflammation caused by ischemic necrosis in the uterine cavity of women. It is best to go to the hospital for a gynecological ultrasound to confirm the diagnosis, and if necessary to remove the necrotic tissue by scraping or hysteroscopy to avoid cervical infections and uterine infections.