Try some 0000 steel wool (really fine) and some hoppes 9, or similar solvent. You can remove the blue with too much elbow grease, but it works great on rust. Some of that red maybe due to the rust that formed started to change the blue color of the finish. But the steel wool and hoppes should get most of the red off, and use the strongest degreaser you have before using the cold blue (I use MEK- methyl ethyl ketone). Any oil or solvent will make the cold blue come out poorly. It should make it hard to notice the damage done until you're looking closely at the gun. I have also found that letting the cold blue sit overnight before wiping it off really helps the color darken to match the hot blue. Wipe gently with a lightly oiled cloth (rem oil or CLP are great for this) and it should look pretty good.
Hot bluing is rather expensive because hand polishing all the small parts, and uneven surfaces takes a lot of man hours to do it right. You might ask if it would be cheaper to just bead blast it and then blue it. It will be a matte finish, but would help hide the pitting with an even consistent color, and save you a bit of money.
Scheme well, scheme often.
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