Black stool is an indication of blood in the stool which could be caused by various things including gastritis or stomach ulcers. If you continue to have this problem I would highly recommend seeing a digestive specialist. Depending on your age, an ulcer is highly unlikely and it is most likely gastritis, an overproduction of stomach acid. Drugs like prilosec and nexium can help cure this problem. You might have to have an endoscopy done which will determine for sure what is up with your tummy.