Menstrual blood can range in color from brown to even black near the end of the menstrual flow, according to WebMD. This blood is darker because it is older and hasn't been expelled from the body as quickly as the brighter-colored red blood most commonly found toward the beginning and middle of the menstrual cycle. This is completely normal and typically does not indicate any problem.
Other conditions could cause a darkening of menstrual blood, and should this change in blood color take place abruptly, continue throughout the entire menstrual cycle or cause other concern, it is important for individuals to speak with a doctor to determine if there is another cause, according to WebMD. Some conditions that may cause the darkening of menstrual blood include a large uterus or an obstruction of the menstrual blood flow, such as benign polyps in the uterus.
During a visit with a doctor, based on symptoms and if a problem is indicated, tests may be ordered to determine the cause of the darkened blood. Typical tests include a vaginal ultrasound, an MRI, blood work or a biopsy. It is important to remember that problems with menstrual bleeding are rarely found to be anything serious, according to WebMD.