yes, you can use the demo, but pay attention to the documentation as the premium features are marked there. Yes, the demo is MIT, so you can include it in your source code, and credit and link to the plugin Itch page is appreciated. BLAST is closed source for obvious reasons so you can't include it in your repo. But as I said to someone else in my Discord recently, you can simply link the Itch page and write in your repo that the plugin is required. Just like a library that needs to be downloaded separately. There's a dedicated page about license in the documentation. (There's a dedicated page about everything in the documentation)
Make a cool game !