I always thought black squirrels we just a local color variation of the basic grey squirrel ... an inherited thing ... not another type of squirrel (like greys vs. reds). Where I grew upi n Western New York, there were greys in the US; and, in Canada (a mile away, across the river), there were lots of blacks. In my back yard in Massachusetts, there was one family of grey greys and another of blacks. Never saw a mixed-color one anywhere.