Even though the so-called black squirrels look totally different than their gray brethren, they are pretty much one and the same. The blacks are melanistic Gray Squirrels - they have one or two special genes that rewires their genetics to produce an abundance of melanins, or dark pigments. While most are coal-black such as the fellow in the photo, sometimes individuals with blond, gray, or even white highlights can be found.
The city of Kent, Ohio is especially famous for its abundance of black squirrels. Supposedly, the original stock of 10 squirrels was imported in 1961 from Canada and they've since spread like wildfire. But melanistic Gray Squirrels occur naturally and most populations have probably long been present and were not assisted by people. The black squirrels are most prevalent in Ontario, Canada and the northeastern United States. One theory has it that the black form of the Gray Squirrel dominated prior to European settlement, when forests were still primeval and their dark coats helped the squirrels better blend with the shady old-growth woodlands. As the forests were opened up and hunting of squirrels increased, gray forms were favored as they blended better with the changing habitats.