Black and jelly-like menstrual blood means that it is retained in the uterus and vagina for a long time, and the bleeding rate is fast enough to cause blood clots, which will be seen as black and jelly-like menstrual blood. Normal menstrual blood is dark red, not black, menstrual blood in addition to blood, but also some of the uterine mucus and cervical mucus, so menstrual blood does not only look liquid, it also contains some solid components. Menstrual blood contains proteolytic enzymes which are non-clotting, but if the bleeding is heavy or very fast there will be blood clots, the jelly-like mentioned above. If the blood stays in the uterus or vagina for a long time before it is discharged, the blood will turn from dark red to black, so some women may find that the blood is black when they have just started their period or when their period is about to be cleared.