I purchased TES Morrowind not too long ago, worked fine on 8.1
As soon as i did the update to 10, 2/50 of my games that dont support 1920x1080 have this black bar issue. Sadly one of those games is morrowind.
really hope there is a fix in the next week or so.. Not enjoyable with two big black bars on either side.
Have you checked the game compatibility with Win 10? Have you checked for patches?