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.
Unfortunately this is something I seem to see quite often on dog forums. People talking about 'detox' and blood and vomit and loose stools as if it's normal. To me it never is and I would never put my dog through a diet just because I thought it was healthier if it clearly wasn't working. I'm not saying that some dogs don't do amazing on raw but I think it just doesn't work for a lot of dogs and there's nothing wrong with admitting that.
To me clean teeth wouldn't be worth it, honestly. You could brush them and likely get similar results.
I'm not trying to be snobby or a downer, it just... I used to be that way, I'd want SO bad for a particular food or diet to work for Jax because *I* liked it better but I had to find what worked for HIS system best. And it just seems like I see time and time again through the years that people on forums are like "oh he's doing GREAT on raw, his teeth are clean, but he's just had a bit of bloody stool, vomiting and excess shedding, but other than that, GREAT!" lol and I know that's not what you're saying, but it just seems to be a common theme.
I dogwalk for a Bouvier who is fed raw and his stools have never been consistent in two years. Sometimes he won't poop for a week, then sometimes he has loose stool 3x in one walk, sometimes it's rock hard, etc.
Oh trust me, I'm just trying to find something to feed Louis that he does well on. I was hoping raw would be the magic bullet. Clearly things didn't work out that way. I'm just tired of the kibble merry-go-round, because I haven't found one that he does well on. He's itchy as all get out, and aaaaaalllllways has loose-ish stool.
Clean teeth are pretty important to me, and from my personal experience, brushing them doesn't even come close to the teeth of a raw fed dog. However, obviously clean teeth aren't a trade off for an upset GI tract!
We're now transitioning to a hydrolyzed protein diet to see if that gives his system a rest. Crossing fingers.