Dark bowel movements can be red, brown, gray or black. The cause of the stool darkening will determine the color; however, most dark stool appears black.
Bilirubin, which is found in bile, undergoes many chemical changes as it passes through the digestive tract. These changes can account for the color of stool, which is usually light or dark brown for normal processes or dark green when chemical changes have happened rapidly.
Duodenal, gastric and peptic ulcers can all cause stools to become black.
Tumors in the abdomen and intestines may cause intestinal bleeding leading to tarry, black stool. These tumors can also cause blockages keeping bilirubin from making it into the stool, which will cause gray, clay-like bowel movements.
When bleeding occurs in the digestive tract, for any reason, stool will turn black due to the chemical changes to the hemoglobin flowing into the intestinal tract. If the bleeding is occurring in the lower intestines, than stools will appear red in color.
High intake of iron from ingesting foods like beets or medicines such as bismuth subsalicylate or Pepto Bismol will harmlessly darken stool.