Black stools are common when ingesting a bactricidal substance such as Pepto-Bismal or an antibiotic that kills the bacteria in the colon. It can also be an indication of intestinal bleeding high up in the intestine and an occult stool sample is in order. Commonly, the stool smears are collected at home, placed in a special packet, and mailed in to the lab.
Generally, if the problem is from antibiotics, replenishing the system with yogurt will return the system to normal.
Different things can cause black bowel. Something you ate, high iron, certain medicines like Pepto Bismol, etc. If you are consistantly having black stools or black tarry stools, I would recommend seeing a doc.