How come there are lots of black squirrels now?

Where I live we used to have a bunch of grey squirrels, then all of a sudden we have only black squirrels, I haven't seen a grey one in about a year and was just wondering where did all the new ones come from. I never seen a black squirrel until about a year about and since have seen nothing but them.How come there are lots of black squirrels now?
Where are you from? I'm from Pennsylvania and I saw my first black squirrel about 10 years ago. I though it was pretty neat because up until that time I had only seen grey or red squirrels. Anyway, A woman I worked with liked to hunt squirrels, she said that they were very rare in Pa but more common further north and in Canada.