There are many reasons for the dark color of menstrual blood, which may be caused by the cold of the uterus, mental factors, weakness of qi and blood, and endocrine disorders. The first thing you should do is to keep your vulva clean and hygienic, and to keep your spirits happy. It is very important to avoid getting cold. In this case, you should also go to the hospital for a checkup, a gynecological ultrasound and a hormone six test, and then make adjustments according to the test results.