Hi.. Pepto bismul contain bismuth salicylate which is used for its antacid properties. However it can react with the contents of the saliva and the gastic contents forming a black colored product which can coat the tongue and the stools dark.. so your stools may be black.. If you are using this product of the treatment of ulcer, you may have to differentiate between what is know as malena, where bleeding in the stomach and intestines can also pass in the stools as dark tar colored stools..