Blood in the feces is not reported as a side effect from birth control pills. Fresh blood in the feces (rectorrhagia) is usually due to hemorrhoids and rarely because of infection (colitis) or tumors in the colon. Black blood (melena) in the feces is due to bleeding in the higher parts of the gastro-intestinal tract (esophagus, stomach and duodenum). In any case, blood in the feces (melena or rectorrhagia) should be taken very seriously and has to be examined immediately! A physical examination and colonoscopy or gastroscopy and some laboratory analysis must be done. You may seek an expert opinion from a gastro-intestinal internist.