Under normal circumstances, menstrual blood is dark red and non-coagulable. If the menstrual blood is black, it is often clinically related to the following factors: 1. Abnormal position of the uterus, such as forward-flexed or backward-flexed position of the uterus, menstrual blood cannot be removed from the uterine cavity in time and becomes stale bleeding when it is discharged, so the color is darker and will be black. 2. 3. Due to pelvic tumor, when the pelvic tumor presses on the uterus, the menstrual blood will not flow smoothly.