Yes, blood in stool = immediate vet visit. He may have salmonella poisoning, he could be having a pancreatitis episode, or even be the beginnings of HGE, which can be fatal without supportive care.
I would not be feeding ANY diet that caused my dog to have bloody diarrhea, whether it is "good for them" or not. If you had switched his kibble and he had a reaction like this you wouldn't continue feeding that brand of kibble, so I'd be putting him back on a low residue, easy to digest kibble until he's back to himself after seeking veterinary care.
With any luck all he'll need is some metronidazole to get him feeling better.

That stuff does wonders.