This is how I would answer your question. I would google blood in vomit. I would research how serious this is and if it requires immediate attention. If you find out that a vet should really say "oh please come down to see us ASAP" then scrap that vet because they should have known that and you shouldn't have to search the net for the answers. But if you find all sorts if info and possible causes and it is usually not life threatening, then assume the vet new this and that is why they were not alarmed.
I know that with humans, red blood in vomit is not a cause for serious alarm, though should be checked out, however black blood(or old blood, will look like coffee) in vomit is a trip to emerg ASAP. For a pooch I would assume something similar.
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Mary
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