Whenever I'm inside a building and trade with an NPC or open up the menu the background becomes pitch black and I can't see through it at all. As soon as I go outdoors, it's clear again. Is it supposed to be this way?

I'm pretty sure the background is supposed to be blurred, not pitch black.

I thought so too, I wonder what it is that causes this. I'm sure it's a mod but I don't have any mods that alter the menus in any way so I have no idea where to start looking.