I use to have a black squirrel that had a white tip on it's tail, I called her "stinky" I use to feed her and all of my little buddies in Stanley park in Vancouver. Haven't seen her around for a while.
I don't know too much about the official species names of all our squirrels ... I know the blacks and greys normally have nothing to do with each other. Once in awhile they may mingle and the offspring can go either way. We have "black" squirrels; "grey" squirrels; and, "red" squirrels (very small, similar in size to chipmunks). In this area it's mostly black squirrels ... but normally they're all black. This particular squirrel is the second one with a white-tipped tail that I've ever seen in my whole life. Occasionally we spotted a black squirrel with a reddish brown tail but that was a couple of years back and I suspect he's a goner now. (was nesting in somebody's roof; people don't like that!)