There could be internal bleeding. Its hard to tell from where, but the color of stool may be a good indicator.Black is high in the digestive tract, could be a stomach ulcer. Reddish is further down, could be ulcers in small intestine or ulcerative colitis. Red blotches or streaks on surface of stool could be bleeding in the large intestine or rectum. This could be bleeding hemorrhoids. You need to take a sample of the dogs stool to the vet.
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There could be internal bleeding. Its hard to tell from where, but the color of stool may be a good indicator.Black is high in the digestive tract, could be a stomach ulcer. Reddish is further down, could be ulcers in small intestine or ulcerative colitis. Red blotches or streaks on surface of stool could be bleeding in the large intestine or rectum. This could be bleeding hemorrhoids. You need to take a sample of the dogs stool to the vet.
http://www.answerbag.com/q_vie...

Sometimes it is normal to have bright blood in stools if the dog has diarrhea,as it's just irritation.

But having diarrhea that long might be a cause of concern-and it might not hurt to have him looked at by a vet-SOMETIMES it's an early symptom of parvo,and if you catch early it can be treated,but don't panic it might be something like what he was eating

The main concern would be dehydration-and sometimes a vet can just give them something to harden the stools.
http://en.allexperts.com/q/Dog...

if your dog has diarrhea or blood in the stool for more than two days, this could be a larger problem that should definitely be discussed with a veterinarian.
http://www.old-dog-treats-and-...
The above has some good remedies.

If the blood is dark dried red go see vet. If it is fresh probably just broken blood vessle.