I had a chocolate lab when I lived in Oakland (Ca). The dog was the friendliest, goofiest guy around, unless he saw a negro (which, in Oakland, is all the time). I've heard of other dogs having this type of reaction, so I never thought much of it. But back then (I think it was early 2002), I lived near Lake Merritt, which is also where the mayor of Oakland would walk every day. He (Jerry Brown) also had a lab and our dogs would play as we walked, but my dog stopped to bark at every black person who passed by. I looked at the mayor and said "I guess you trained your pup to not react to threats".
I think he figured out what I meant after I left.
I had a chocolate lab that was the same exact way. Friend of everyone, as long as they weren't black. He'd turn up his lip, the hackles would go up on the back of his neck, he'd plant himselft directly between me and the black person and just let out the deepest, scariest growl a dog could possibly muster up. I couldn't take him anywhere where we might cross paths with black people. It wouldn't have been safe for any of us.
