Dark-colored blood towards the end of a menstrual cycle indicates the expulsion of older blood and is not a cause for concern, according to WebMD. There are some instances in which this is an indicator of other serious health issues.
Because older blood is not as freshly oxidated, it is typically darker in color and can be dark brown or almost black, explains WebMD. In these circumstances, the dark color is not an issue.
There are some situations in which darkly colored bloody discharge indicates a serious problem, states, Phaa.com. These conditions include retained menses as a result of an imperforate hymen or cervical stenosis, a miscarriage, a bacterial infection or an infection related to a sexually transmitted disease, or cancer. Should darkly colored bleeding occur at any time other than at the end of a menstrual cycle, medical attention is advised.