If anyone still sells it at gunshows see if you can find a bottle of RB-17. Best stuff I've ever seen for removing rust from almost anything. Put the gel on the rust spot, wait a minute or two and go over it with some 000 steel wool. Rust comes off and if your careful will not hurt the blueing too much.
Thanks, I'll have to check them out. A brother-in-law has a gun that is BADLY rusted from years of neglect while being stored in a fleece lined rifle case. Might be interesting to see what it would do although I think the rifle barrel is totally trashed.