Kano Kroil ( available from Brownell's ) is a first rate penetrating oil product that will soften rust and grungy varnished oil and dirt and will even remove barrel leading. Saturate the surface and let the product work and scrub with a stiff tooth brush. It may take a while but it will work. After Kroil does its work then you can clean with denatured alcohol ( hardware store ) to remove the Kroil. You can also blow it off with carburetor or brake cleaner but be sure you have adequate ventilation as the stuff is hard on one's liver. The trick is to remove grunge chemically not mechanically as any abrasive product will remove the blue to some degree. Keep in mind that guns of this vintage may not have a particularly shiny
blue to begin with. Over the years I've had excellent results with Birchwood Casey's Barrier as a preservative and I very much agree with other fellas that a good wax is an excellent preservative and shine. You can clean the grip panels with Windex and a tooth brush followed by a light coating of tung oil.
Good luck with your project.