Normal menstrual blood itself has different shades.
Before menstruation: there is light pink or light brown secretion in the amount of paper towel/pad.
Menstruation day 1-2: The bright red or crimson menstrual blood gradually increases.
Menstruation Day 3-4: The color gradually darkens, showing brown or black menstrual blood.
Day 5-7 of menstruation: the color gradually turns light brown.
Normal menstrual bleeding lasts for 2-8 days and occurs every 2 1-35 days, while black menstrual bleeding is a part of normal menstruation. This happens occasionally during menstruation, so you can observe it first and don't need special treatment. But if you don't get better for three months in a row, you should see a doctor in time.