This character showed up while we were feeding another squirrel, a normal grey female. They quarreled a bit and eventually the black squirrel, who didn't yet know about our nut supply, ran off. I hope she comes back!
Although we occasionally, rarely, get black squirrel visitors, I've never seen one with a red tail before.
The black squirrels are a darker variant of our normal Eastern grey squirrels, not a different species. They seem to have spread southward from Palo Alto, but in recent years have become fairly common in nearby Campbell. This visit was in December of 2012.