Bloody diarrhea is loose, watery stool that is red or black in color and appears to contain blood. Bloody diarrhea typically signals a problem in the digestive system, such as bleeding. The color of the bloody diarrhea can indicate where in the digestive tract the bleeding is occurring. While black bloody diarrhea may indicate a problem with the upper portion of the digestive tract, bright or dark red stools often indicate a problem with the lower digestive tract. Possible health problems that may cause blood diarrhea include blood vessel abnormalities, tears, bleeding in the stomach, inflammation in the stomach, a foreign object, or insufficient blood flow to the bowels.