Blood in the stool can come from anywhere in the digestive tract, such as the stomach or intestines. Depending on where the blood is coming from and how fast it is moving, it may be bright red, reddish brown, or black like tar.
A little bit of bright red blood on the stool or on the toilet paper is often caused by mild irritation of the rectum. For example, this can happen when straining hard to pass a stool or if a hemorrhoid is present.
Certain medicines and foods can affect the color of stool. Diarrhea medicines (such as Pepto-Bismol) and iron tablets can make the stool black. Eating lots of beets may turn the stool red. Eating foods with black or dark blue food coloring can turn the stool black.