I would see a doc asap because black stools, if not caused by pepto bismol as also said above, could be a sign of bleeding from the upper part of the colon. Polyps, hemmorhoids, and tumors from the last part of the colon cause red bleeding. Good luck and hope its all well.
I had this before also. I got really worried when that happened. I went to see my doctor and her first question was have you taken any peto-Bismol or anything similar in the last little while. I had and she said thats what it was from i felt pretty stupid but hankful it was nothing. For safety i still would go see your doctor about it. Good Luck