Good News! we may have figured out your graphical glitch issue! This appears to be a WebGL issue. The textures for the banish trick are HUGE (> 9000 pixels. blame bad jam design decisions!). WebGL has a value called: Max Texture Size. Often this is set to 16384 pixels, but we've noticed that sometimes this value can dynamically change. You can see the current value for your browser by heading to: https://browserleaks.com/webgl. If the value is 16384, you'll be able to play OK. If the value is 8192, then it won't load properly. The weird thing is, we've seen it at 8192 (causing the black box issue) then if you close all browsers and applications and open a fresh browser, it goes back to 16384 and the game plays fine, so while The Internet says this is a value set by the GPU, it seems like that value can dynamically change.
So one solution is to just close all applications and reopen everything. Or play a downloaded version.
Anyway, sorry for the issue back when you ran into this but at least you'll know what the issue might be if you run into anything similar in the future.