The passage of tarry black motions is usually due to bleeding high in the gastrointestinal tract, such as the stomach or duodenum the blood has been altered in colour by the action of the stomach acid.  Black stools may occur if the bleeding arises from the colon especially on the right side near the appendix and its slow passage through the colon allows time for its colour to be altered from red to black.  It has the same significance as haematemesis (vomiting blood) and warrants early investigation and replacement of blood loss if necessary.