Blood in diarrhea stool indicates bleeding in the digestive tract (gastrointestinal tract), usually in or below the stomach. This can be caused by inflammation, irritation, or infection. Bloody stools may occur as one large loose stool or several stools containing a moderate amount of blood. If your child is toilet trained, there may be enough blood to turn the water in the toilet bowl red.
Depending on where the blood is coming from and how quickly it is moving through the digestive tract, the blood may be bright red, reddish brown, black, or tarry. The blood will be mixed in with the stool.
Certain foods and medicines also can change the look of your child's stool. Taking medication containing bismuth subsalicylate (such as Pepto-Bismol) or iron tablets can make the stool black, and eating lots of beets may turn the stool red. Some food colorings also can change the color of your child's stool. Eating foods with black or dark blue food coloring can turn your child's stool black
WebMD Medical Reference from Healthwise