Black Squirrels at Princeton University:

Princeton University has both gray and (about 20%) black squirrels. There are some nearby areas with blacks also (Cranbury), but somehow the blacks keep to the Princeton area and have not expanded at large into central New Jersey. I've watched these rare squirrels for many years and have been fascinated by their persistence and lack of territorial expansion. At last, like any sensible person, I visited the web to see what I could find about them.

Here's a web page with some general information about black squirrels. A page discussing "Princeton Myths" has this to say about black squirrels: "Black squirrels may be ubiquitous at Princeton, but they are also perfectly natural. Black and gray squirrels are merely different color morphs of the same species, the eastern gray squirrel (Sciurus carolinensis). Populations of black squirrels appear naturally in other places in the Northeast, particularly in isolated populations such as those in city parks, and they have been around for a long time; in 1655 David De Vries included squirrels "black as pitch and gray" in his list of the wild mammals of New Amsterdam."

Do genetics determine the persistence of black squirels? Here's a discussion. A quote: "This mutant of the gray squirrel resides primarily in northern climates. Biologists surmise that the black fur more readily absorbs the rays of the sun, thereby keeping its owner warmer during cold winters. Selective genetics has given the black squirrel this survival advantage. One of the reasons they seem to be more abundant in cities is that their black coloration is more readily spotted in rural areas by predators, primarily birds of prey."
I'm guessing that the black coat's advantage/disadvantage may account for the squirrels' survival in Princeton without heir expanding to all adjacent regions. There is still some forest and farmland in central New Jersey, where their black coats would mark them for predators.

Not surprisingly, Princeton University has studied its black squirrels. Here's a page that goes into genetic details. That page includes Professor Henry Horn's calculations to show how, although black is the dominant gene, it does not spread to dominate the entire squirrel population.