The first reason is what we’ve stated above. It could be the beginning or the end of a period when blood flow is typically slower. This allows for the blood to have more time to oxidize. So it can become so dark it eventually appears to be black blood.
Stuck foreign object
Another reason is that something could be stuck and hampering blood flow. It could be a foreign object like a condom, contraceptive device (cervical caps, diaphragms, or vaginal rings).
It could also be hematocolpos or retained menses. This is when menstrual blood can’t leave the vagina. Instead, it collects, while slowly oxidizing and getting darker. It’s often the hymen, the septum, or surgical complications that could cause this. However, if you have not been diagnosed with this condition, it is highly unlikely.
Rarely, cervical cancer
The next reason is very rare but worth looking into. This is especially true if you also experience irregular periods, metrorrhagia, and bleeding after sex. Black period blood could, in very rare cases, be an early symptom of cervical cancer.
Early-stage doesn’t really have any distinctive signs but if you have a history of cancer, you might want to err on the side of caution and get it checked. At the later stages, the symptoms are bleeding or pain during or after sex, fatigue, longer and heavier periods, pelvic pain, and swollen legs.
Postpartum lochia (discharge)
If you had recently given birth, black blood might be postpartum lochia which is a perfectly normal form of bleeding that occurs around 6-8 weeks after childbirth. A lot of blood has accumulated and eventually, the body expels dark or black blood that eventually lightens as the bleeding continues.
If you are less than 5 months pregnant, black blood could, unfortunately, be an indication of an early miscarriage. The body might go into a missed miscarriage and still present symptoms of pregnancy so the best course of action is an ultrasound to precisely determine what has happened and set your worries to rest.