Damn I'm so sorry I didn't see the notifications for this. If you still can, do you know if you have a Vulkan GPU? The windows version doesn't need Vulkan, but trying to run the Windows version with Proton will cause the the background to be a black screen. One way to circumvent this would be to have an option for a non-transparent background...
We could try to see what happens when you open the game with the console log. My first guess is that, a Vulkan related issue, but it could be anything. I'll talk with my friend who's helping me with Linux to see what we could do about it. Sorry about that, if this continues I'm sure itch will offer a refund.