You can use bluing remover, bead blast, shot peen, sand blast, sand paper, ammonia fuming, nitric acid bath, or whatever you want. Or just pain it, if you are so inclined. Degrease it first.
If you're going to paint it you can just degrease really well and apply the paint over the bluing.
The remaining bluing will aid in rust prevention.
The best pre-paint treatment is to bead blast the metal. This leaves a rough finish that gives the paint a "tooth" to bond to and greatly improves adhesion.
