The black or green stool can last for several days, but it is harmless. This change in stool color is not a cause for concern unless you have stopped taking the bismuth subsalicylate and the unusual stool color persists.
Sources:
American Society of Health-System Pharmacists, Inc. "Bismuth Subsalicylate." MedlinePlus. 01 June 2007. 23 Oct 2008.

