Hi, Nick,

I was born and raised about 20 miles from Ann Arbor, MI. You're right, the grey squirrels there aren't so much grey as they are bronze. Further north in Michigan, (in the Lower Peninsula) there's a species that's solid black. I don't know enough about them to say if they are merely a color morph or if they are a definitely distinct species. They're nice looking squirrels, though.
Michelle