I don't know whether oreo cookies have affected your motions. However, the medical name for black tarry stools is MELENA (you can look it up on a google/yahoo posting etc). Melena arises from intestinal bleeding (usually in the small intestine).
If you have melena you should seek prompt medical advice since rectal bleeding (whether "loose blood" or via melena) is never normal.
regards
Morecambe
Can my question be answered. what causes black tarry stool