Is black stool a symptom of colon cancer or ulcer?

Depends: There are many causes of black tarry stool. The common issue is bleeding in the intestine. Bleeding may be due to ulcers in the stomach, or intestine. Oral iron intake may produce dark stool. Colon cancer is one of the causes of bleeding in the intestine. It would be prudent to see your doctor to determine the cause of dark stools, especially if it is due to bleeding in the intestine.

Ulcer: Bleeding from a tumor of the colon results in the presence of blood in the stool. This blood is for the most part red, but may have signs of some decomposition. When blood is released from a stomach ulcer, the acid that is present breaks the Hgb down to result in a black appearing stool which is not seen is colon malignancies.

It can be: Black stool is a sign of blood within the GI tract visually from the upper GI tract (stomach and small bowel). It is ALWAYS worth while to get checked out by a gastroenterologist to be certain.