I thought it had to do with created types, etc. Just like when the old Norton would build a boot disk. I forget the version, it was old, but it left out the finder too. It was simply because there was not room. As far as I remember any app can replace the finder. After all, the Finder is an app. The system simply looks for something that looks like the finder and launches it. Another app can fool the system.

I just don't recall the details. I would have to fire up ResEdit and look around. As for blessing, should be the same. Move the app out, then back in, then close/open the system folder - or some variation of that.