I had a gunsmith blue a 1911 slide for me years ago (he nickel plated the frame & did a great job!) Once I got the gun back home I noticed the slide was sort'a splotchy looking with plum coloring. I called the fellow and he said, "No biggie, heat your oven up to about 250°-300° and bake it for an hour or so. When it comes out, scrub it down real good with a wadded up newspaper, and it should go away." Danged if it didn't! He's an old fella by now, but I hear he's still at it. I don't know the chemistry behind what I did, but I know it worked. It's a nice even blackish blue now, and that was about 30 years ago.
My 39-2 is a shade lighter than the alloy frame, but not bad. It's a '78 year.