If you're not on iron, get checked out by the GP, as it could be a stomach ulcer or some such bleeding high in the gut.
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"Iron supplements, taken alone or as part of a multivitamin for iron-deficient anemia, may cause stools to be black or even greenish in color. Foods that are dark blue or black in color may also cause black stools. Substances that can cause false melena are:
* Black licorice
* Blueberries
* Iron supplements
* Lead
* Bismuth (Pepto-bismol)
A physician should be consulted immediately if black stools can not be attributed to a benign cause such as an iron supplement or a food."</i></font id="blue">