Will contrast in a cat scan cause black stool?

No.: The ct contrast is dilute, a small amount of either barium or iodine in a large amount of water. It shouldn't be visible in your stool. Black tarry stools are a sign of internal bleeding. It could be a sign of colon cancer or other important problem. Ask your doctor.

No but see your doc!: The injected contrast is clear and mostly is eliminated by your kidneys. The oral contrast is either clear or white. Black stool could be from bleeding, possibly from your stomach or proximal small bowel. See your health care provider asap!