itch.io

I'm not sure that's easily fixable. It's well known that larger your raft, the more demanding the game becomes and longer it'll take to load. If your raft is humongous, you could try waiting 15 minutes to see if raft  is rendered eventually.

https://support.redbeetinteractive.com/hc/en-us/articles/360012981652-Black-screen-when-starting-a-game

You could also try this workaround:

The screen stays black when joining someone
Lower your graphic settings before joining and then return them to your normal settings after you loaded in.