Tarry stools are not necessarily a sign of cancer. If there is bleeding in the upper gastrointestinal tract and the amount of bleeding is large, the patient may pass tarry stools during bowel movements. How does tarry stool form? Tarry stools are usually black and sticky. The reason for this is that the blood cells remain in the intestinal lumen for too long and undergo a putrefaction reaction, which makes the blood look like black tar, hence the name “tarry stools”.