Where are you hunting..??..
Black squirrels are just a melanistic color phase of the grey squirrrel... Same species, different color phase.. In some places they are quite rare..In other places they are common or even dominant...
Examples...Where I live in western NY, they are fairly common, but not exactly an everyday occurance... I have seen as many as 2 or 3 in one day of hunting, but often I might hunt a couple of weeks and see hundreds of grey squirrels but no black squirrels..
In an area where I used to hunt deer in northern Mckean Co. Pa, black squirrels were very common...I would see them along with grey squirrels in about a 50/50 ratio...
In Prince Edward Co. Ontario, where I hunt gobblers every spring with a buddy who lives there, nearly ALL the squirrels are black.. I can't remember ever seeing a grey one up there..