Some other random Blitz related repos:
https://github.com/earok/GloomAmiga <- Gloom is an ASM game, but the tools were written in Blitz and thus the source is available. They're all tokenised though so you'll need to open them in Blitz to be able to read them.
https://github.com/earok/MrZen <- Mr Zen is a platformer that was sent in to the Amiga Format Blitz Basic competition. Source is provided with permission from the original developer.
https://github.com/earok/SpaceMan <- Spaceman is an isometric shooter that was also sent in to the Amiga Format Blitz Basic competition. I haven't made contact with the original developers so I'm happy to take it down if they object. I have made a couple of my own changes to the source and checked it in (if you want to see the original source, you just have to go back to the initial commit)