Probably is: Stool color change based on your diet and intestinal transit time and absorption. Dark brown and green stools do happen and does happen once in a while. However, if it is truly tarry and black, get it checked for blood since it will suggest bleeding above the stomach (ulcer?). Taking things like a lot of blueberries, iron and pepto bismol (bismuth subsalicylate) can make your stools black as well.
Answered 11/28/2017
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