Acid reflux usually doesn't cause bleeding to the point that black stools would arise. First, make sure that there is blood in the stool and check for anemia. See a PCP or GI for work up.

See your doctor to make sure you don’t have a condition known as peptic ulcer. That’s the most likely cause and it’s usually an emergency. Have you been taking peptobismol lately? That can also cause black stools.