If it is just a few shiny spots on the corners I would simply use a cold bluing solution like Birchwood Casey's Perma Blue. Degrease the part and follow the instructions on the container, applying it only to the areas you want blued. If the surface area is larger, I would suggest either having it professionaly hot blued, or looking at some of the alternative finishes offered by Brownells (call 515-623-4000 for a catalog) they are also on the web, but I forget the URL.