There are many causes of why blood appears in a dogs' stool. When bright red blood appears on the surface of the stool, the lining of the large intestine could been scraped by something sharp that the dog ate. Parasites like whipworms may also cause blood in the stool as they irritate the walls of the intestine. Tarry, dry, or dark blood mixed with the stool is a sign of small intestine problem. An inflamed large intestine can also cause blood on the stool.An infected anal sac may also cause bleeding. If your dog manifest any of the following symptoms, you should see your veterinarian as soon as possible. Blood in the stool are tarry or black. Not eating for more than 24 hrs. Pain in the stomach when you push it, bloated abdomen and vomiting. Hope this info will help you. Take care and good luck.