Sitting in a holster in a truck is bad, coupled with the fact that you live in a very humid area. Then we take into account that the truck gets very hot inside and then you turn on the a/c and get it cold which = condensation.
Gun blue is probably the least protection you can put on a metal and you have to keep an oil barrier over it. I generally use a sheepskin treated with
RIG to wipe firearms and it does a pretty good job of creating a moisture barrier over the metal.
0000 steel wool will probably do a good job of removing rust, but cold blue does little to protect the metal. It will help the appearance though.