Thank you! Those black boxes are definitely not intended. I haven't been able to replicate this yet, so if you wouldn't mind sharing the Windows version, graphics card, and whether windows transparency effects are on (Settings -> Personalization -> Colors -> Transparency Effects). Also I'm curious what happens if you go to the game settings and turn Windowed Mode on.
If you'd rather we have a feedback form here or hop in our Discord and we can chat more. Thanks for playing!
