Black blood on the first day of menstruation is caused by insufficient qi and blood. At this time, the amount of menstrual flow is relatively small, and the menstrual blood stays in the vagina for a long time, and in the acidic environment of the vagina, the menstrual blood turns into black color under the action of certain enzymes in the vagina, which is a normal phenomenon. Under normal circumstances, the first day of menstruation blood is relatively small, the second or third day of the blood began to increase, menstrual blood volume when the bleeding speed increases, menstrual blood is dark red. When menstrual blood becomes black, it is considered that there is insufficient qi and blood or menstrual disorders. Normally, it is necessary to benefit the qi and nourish the blood, and during menstruation, medicines can be used to invigorate the blood and promote the contraction of the uterus. During menstruation, you should not eat spicy food and cold food.