Where do the black squirrels in England come from ?

I read on the news that black squirrels are common sighted in Hertfordshire and Cambridgeshire. Are they that common ? Where do these black squirrels come from ?Where do the black squirrels in England come from ?
It is just a color variation of the Grey squirrel...sort of like a black panther found in some of the big catsWhere do the black squirrels in England come from ?
Squirrels come from the trees.Thats where they live.