As you can see the list of possible causes for blood in a dog's stool are several. Sorting through all the causes and finding the actual culprit requires diagnostic testing. For example, your vet may start by taking a look at a stool sample through a microscope to see whether there are any parasites or protozoans. If nothing is found, blood tests may look for any underlying systemic issues. Then further tests may be needed to sort through more possibilities. A vet visit is therefore needed to get at the bottom of this.