Blue buffalo is a good food, But.. Honestly.. At least here.. banfields are AWFUL!
Most mis diagnose, and will charge a arm and a leg for nothing. But, that is here. LOL I Wouldn't touch one if it was last vet in a 20mile radius.
Do you have any friends with dogs near you? That you can get a personal reference for there vet? But, I too.. Would have it skin scraped. I am very surprised your vet didn't do it.