Darkly colored stools are of no medical significance, but ones that are truly black demand an explanation. Sometimes a black stool can be caused by the use of Pepto-Bismol, charcoal-containing pills or iron supplements. But if these are not in question, a truly black stool is likely a sign of blood loss in the upper gut, usually the stomach or duodenum. The most likely causes are peptic ulcer and gastritis, but other possibilities exist.
Black stools due to upper GI bleeding known medically as melena are often sticky in consistency. They are sometimes described as "tarry" for this reason. They also have a fairly distinctive odor. The black color is due to the degradation of blood within the bowel by bacteria present in large amounts in the colon. It can easily be proven that melena is due to bleeding by performing a stool Hemoccult (or "guaiac") test on a sample. Home testing kits are available at most drug stores. If symptoms of rapid blood loss such as dizziness, pallor or palpitations accompany the onset of melena, medical attention should be obtained at once.
Also, it's worth noting that if blood loss from the upper gut is very fast, the stool color can be maroon or red, a finding usually associated with lower bowel (colon) bleeding.