The presence of black blood on the 1st day of menstruation can be caused by more reasons: First, when there is pre-menstrual period, due to environmental, stress and mental factors, eating spicy and stimulating food, including the stimulation of alcoholic food, including mental stress, can also cause a certain change in color when the 1st day of menstruation comes. Second, when there is endocrine hormone imbalance in the body, the endometrium will become thinner due to changes in hormone levels, and black blood will be present when menstruation occurs. Third, when the progesterone in the body has not yet completely withdrawn and the endometrium has just started to peel off, black blood will be present due to oxidation when it enters the vagina. It is recommended to observe the amount on the second day, as long as the amount on the second day is normal, it will not affect your health. However, if it occurs continuously, it is recommended to check the hormone level on the 2nd to 5th day of menstruation to rule out any abnormalities.