Wideblock (also known as 16:9 beatblock) is a simple QoL Lovely mod that gets rid of those black bars when you're in fullscreen.
To install Wideblock, you will need to first download Beatblock+, Lovely, and a copy of Beatblock.
Once you have installed Beatblock+, Open %appdata%/Beatblock/mods, Extract the folder from the zip and put it there.
Once done, just run Beatblock and it should apply.