What causes grainy dark stools in adults?

Diet & bacteria. The color and texture of stool is most often affected by diet and the activity of normal bowel bacteria. The only colors that are worrisome are red, black, or white.
Stool appearance. The color of one's stools often has little significance. The exception is that of having dark, nearly black colored stools. Bleeding in the upper GI tract may cause black tarry stools. Bleeding may be caused by peptic ulcer disease or esophagitis, for example.