Is this specifically made to be used on Blued firearms? Since gun bluing is a form or oxidation, if its not specific as to what it was made for, I'd be reluctant to try it since it might do more damage than already done. I have some luck using Kroil on rust spots, keeping the spot soaked for a couple days then using a coarse cloth such as a washcloth to scrub the loosened rust.
evapo rust will flat remove bluing.. I use it quite often for that very purpose. then polish the part.

0000 steel wool and some light oil will remove most surface rust with ease
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