Hello, Thank you for your query. I am going to answer on the basis that you have black menstrual blood and not black stools (based on you query). Old clotted blood is blackish and can occur at the beginning or end of a cycle. Tar like blood, however signifies some other clotting problem or a cyst (endometrial cyst) *do you have any pain whole menstruating? *any clots that pass? Any smell to the discharge? *are you on any supplements, especially iron? *do you have any other medical issues or take any medications? *do you have children? I recommend : -an OBGYN consult -an ultrasound abdomen -blood tests (complete blood count, clotting profile, serum ferritin, liver & kidney function tests) -urine analysis. - stool analysis (if black stools are present). Based on this we can arrive at the cause. Please see a gynecologist as soon as possible. Hope this helps. Please let me know if there is anything else I can help you with. Wish you good health