Eastern Gray Squirrel (dark phase)

I think it's because it is a mutation that is locally common in some populations (like wise the white mutation seems common in a few small populations). It is likely that most of the grey squirrels that were introduced to Europe were of populations that did not have this mutation... and considering the over all population of them, I'd say it's actually rather rare in the over all population. Kinda like how the population of the Kermode black bears have a fair frequency (1/10) of a white, non-albino mutation, but outside of that population, white black bears are rare.

Interesting - we have grey squirrels in Europe (Sciurus carolinensis), introduced from USA in the 19th century, but I've never seen a black phase over here. They have driven away most of the Red Squirrels (Sciurus vulgaris) from the UK but these are still plentiful in the parks near where I live in Spain.

We have these guys all over Michigan - they're the black color phase of the Eastern Gray Squirrel. I actually raised an abandoned youngster of this variety....they're fascinating animals!