Hi XXXXXXXXXXX~
Is he a chewer?Do you give him bones, rawhides, ect?
My best guess, without being able to see the dog, is that he maybe either loosened a tooth or cut his mouth on the rawhide. The blood could have been from that, and then he swallowed some, which could cause the blood you saw in the vomit.
Since it was only one episode, I think I'd just monitor the situation for tonight...if it starts again, or if he seems really lethargic or pale, then you should have him seen by an emergency vet in your area.
Even if you have no more episodes this weekend, it might not be a bad idea to have your vet take a peek at him on Monday..just to check for loose teeth.
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