The blood is a concern.
There is a bad condition that involves bloody diarrhea, abbreviated HGE, that can strike hard and fast and can be severe. In many cases there is a correlation to clostridium perfringens, but it's not a definitive correlation. My girl had C. perfringens and it didn't show up on a fecal, though it did how up on the $200 test - it was producing toxins. This may not be what's wrong with your pup, but just a possibility.
I would feed a bland homecooked diet for awhile. I would alo want a probiotic that contains sacchromyces boulardii, which I protective for clostridia. For my 20 pound girl, I use this: http://www.renewlife.com/ultimate-fl...5-billion.html
, together with the mercola probiotics, and apple pectin (which helps good bacteria grow and deters bad bacteria). Some l-glutamine might help repair the digestive tract, but if there are bad bugs in there, it doesn't affect those.
You may need antibiotic therapy if it doesn't continue. You may need a second opinion with a different vet.