Dark brown bleeding for month

Hi,
Welcome to ehealthforum.
Change in bleeding pattern along with dark bleeding could indicate possibility of pregnancy (women are known to get pregnant while being on BCP’s). If you were not regular on the pill before, chances of pregnancy could be present. Look out for other symptoms suggestive of early pregnancy. Dark periods are also suggestive of old and oxidized blood coming out. Another possibility could be infection that could cause dark bleeding, consider going ahead with tests done to rule out infections.
Consider meeting your gynecologist for a thorough examination and blood beta HCG level test to confirm/rule out pregnancy. And if infection is detected, then get your partner also treated simultaneously and avoid having intercourse while being on treatment to prevent chances of re-infection.
Last but not the least it could be your birth control pill itself that is causing the dark bleeding as breakthrough bleeding (a common side effect of birth control pill use).
Take care.