Usually changing your dogs diet shouldn't cause blood to be in your dogs stool but if the change in the dogs food caused the dog to become constipated then the dog could've torn it's intestines when straining to poop which then caused the dog to bleed when pooping.
If the blood doesn't clear up in a day or so then you should take the dog to a vet since the blood in the stool could be caused by worms, cancer, Ulcers or other intestinal problems that can usually be treated and fixed by a vet.