There is a race of melanistic Grey Squirrels with a gene for black fur (above: jet black or very dark grey-brown).
See more below.
Above is an albino Grey Squirrel, showing pink eyes and claws as well as totally white fur.
Albinos occur naturally but rarely.
Occasionally Grey Squirrels will have white patches on their fur (topside, since the underside is normally white)
or be piebald (large expanses of white and of grey).
Streaky stayed a couple of seasons in my garden.
Son of Streaky had less white.
Several populations of black Grey Squirrels (Eastern Gray) exist in the US, Canada and in southern UK
(north Hertfordshire, Cambridgeshire). Because the black gene is dominant, they are increasing their range at the expense of greys.
Many of the "black" Grey Squirrels are dark charcoal grey with a jet black tail, the latter without the
white edging prevalent on normal-coloured Greys. Some have fully black fur as well.