Public land close to the city will be your best bet. As mentioned they thrive in the city with few predators. That and the "black" gene mutation is more easily passed between individuals in confined areas like backyards.
I only shot one on public land and I skinned it to mount and never did. Even more rare than the black squirrel is the blonde and the albino. No joke you can google that if you like.
In parts of central Wisco the black/grey ratio is almost 50/50....
I will never understand why some people want to shoot something BECAUSE if its rarity....sounds very backwards to me and a very antiquated reasoning, even if its only a squirrel.
I've got several in my neighborhood, I often see people pulling over and stopping when they see them....they do look much cooler than the typical grey ones.