In some cases black stool indicates bleeding in the upper intestinal tract or stomach, which requires prompt medical attention, according to Mayo Clinic. Other possible causes of black stool include black liquorice, iron supplements, and medications such as Pepto-Bismol and Kaopectate.
Stool that is black or red has some harmless causes but in some cases indicates the presence of blood, which is cause for concern, according to Mayo Clinic. Light-colored, white or clay-like stool indicates a bile duct obstruction. Stool that is yellow and greasy or smells foul indicates a malabsorption disorder such as celiac disease. Bright-red stool normally indicates bleeding in the lower intestinal tract.