What cause black menstrual blood with odor. really bad odor like something is dead?

Oxidation: If you cut your finger cooking at first bleeding would be bright red. As the iron in blood oxidizes it becomes dark red then forms a brown scab as it oxidizes further. The same holds true for vaginal bleeding. The blood oxidizes during transit from the cervix to outside. It gets darker, then brownish black during this transit depending on how long it takes. It oxidizes. Hence, too, the odor.