Black period blood is usually just a sign that old blood is leaving the uterus toward the end of a woman's menstrual cycle, according to WebMD. It is likely not a cause for concern but can indicate in some cases that a woman has an enlarged uterus or menstrual obstruction.
Pregnancy often leaves a woman with an enlarged uterus, which allows for more blood to collect in the organ once a woman's period returns after giving birth, explains WebMD. This gives the blood time to change colors before being expelled. Some women may develop growths within the uterus that block the flow of menstrual blood and allow blood to turn dark brown or black.