No, it's not a junco.
I saw this bird very briefly this morning as it landed in a small serviceberry tree and proceeded to hop around the garden. The area it was searching was a patch of blue lobelia that was let go to see last year (if that helps).
It reminded me of a chickadee in that it had a rounded, puffed-out chest. It may have been slightly bigger than a chickadee, but not much.
The bird had a jet black face and throat and this colour continued down the sides of the bird creating a thick band that separated the belly from the wings and back. The belly was bright white, no markings that I could see. The crown, back and wings appeared to be slightly less dark than the face, perhaps a slight grey or blue tone, but still very dark. There was also at least one bright white wing band.
This is all I could tell before it hopped away and I grabbed my book (Birds of Ontario, Lone Pine field guide), which did not have it listed anywhere.
I'm in Sarnia, Ontario, about 1km from Lake Huron, surrounded predominantly by mature oak, maple and spruce.