Thick black period blood occurs when blood gets old and it is not removed from the body early enough. This is a common color change so there is no need for a person to be worried.
Many women who experience thick black menstrual flow consider this to be normal. However, if one has regular heavy flows, paying a visit to a doctor might be necessary for a diagnosis, as stated by WebMD. If the condition persists for a longer time, it can lead to anemia in some people, consequently causing weakness. If a person notices unusual changes during her periods, she should seek early medical attention.