You can't see it because this is a photo from a book that was released back in like 2013 with a ton of various images that more better utilized the colors.

Example:

OP's image must have been stolen and reuploaded again and again and again to get us where we are today.

There's a tiny community in Wisconsin that has the Delco accent. It's there.

Why is blue in the key when there's no blue on the map?

Probably a city that is too hard to make out with so few pixels. I would assume Boston, because it would be Boston.

Or maybe they included it because some areas have marry pronounced differently, but forgot to include one for Mary being pronounced differently.