does anyone know of a tool for diffing a blueprint?

I have a problem I have something that is not working and I need to compare to projects that are done in blueprints and see what is different to find the cause

Well, you should work with SourceControl anyway. For example Perforce. Even if you setup the Perforce Server to be on a second Hard-Drive (or the same, although that’s still risky) instead of a real V-Server,
you will have revisions of your files, so if something breaks, you can go back. This also comes with an inEditor version (no plugin needed), which lets you diff 2 Blueprint revisions.

But i have no idea how you could do that after the project is already damaged, sorry.

I have it and it would not have helped in this situation