OK, the nature nerd in me has been activated... cool stuff I think:
Squirrel coat pigments run the gamut from white to black, with various browns as intermediate. These come from varying amounts of two melanin types: one black, the other a reddish-brown. "Black" squirrels can have varying amounts of either and the genes controlling these show incomplete dominance, meaning they can blend.
These pigments can be seen on "normal" squirrels in the banding found on each hair. Then there is the white belly. So... you can potentially have all black, black-brown, brown, reddish-brown, blonde, silver, and (non-albino) all-white squirrels. You can also have pie-balds -patchy ones- of the above colors. Sometimes it's only the tail that varies. And there are albinos too.
Luck of the draw and natural selection give us the squirrels we see. Black squirrels are more frequent in the north, grey ones over most of the range, and the exceedingly rare all-white ones found where protected from predators in towns.
I used to do a lot of squirrel hunting -loved it in fact- and took many Grey and Fox squirrels. I've only taken two "black" ones, one more reddish-black than the other. In Colorado we have Abert's, or Tassel-Eared Squirrels, which are most commonly black where I live, but a pretty grey, red, and white to the west. I don't hunt Abert's as they are not very numerous, their abundance patchy in time and space essentially following Ponderosa Pine seed production.
My fly-tying kit contains squirrel skins (grey, fox, red) but I don't think I saved the black skins because it's the variegated patterns of the normal coats I like most.