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.
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.