Okay, I've had this problem before.
First, right click Blockland in the Steam library, then click Properties.
In the window that shows up, go to the Local Files tab and then click Browse Local Files...
You should see a few folders, a console.log, cache.db, and the Blockland file.
Go to the config folder, then the client folder, and open prefs.cs in whatever text editor you have.
Scroll down to the bottom and try to find $pref::Video::fullScreen. Set that to 0, and try to open Blockland again.
If that doesn't work or if it's already at 0, then I have no idea.
