The most common tree squirrel species in northeastern North America is Sciurus carolinensis (the eastern grey squirrel). Typically, these squirrels are, well…grey! But in a few of the places I have lived, a more unusual squirrel fur colour has dominated.
This dark morph is called a melanistic form. The allele for the dark fur colour is an example of incomplete dominance, meaning that a breeding pair with one black adult and one grey adult will yield 25% black offspring, 25% grey, and 50% brown-black…you know, if they follow Mendelian inheritance. (Which fur colour usually does, but I’m shooting from the hip here, so humour me.)
Some hypothesize that the darker colour is more effective in colder climates (because it absorbs more heat), but I have not found a scientific study to back up this claim. Certainly, the squirrel is low on the food chain, and its camouflage is probably crucial to its survival.
And just for good measure, here is our grey grey squirrel friend.