David, Sig Sauer and Kimber are using PVD coatings on some of their guns. Some of them are rainbow like and I've seen one in electric blue. Here's a link to one of Sig's guns with a rainbow finish.
http://www.sigsauer.com/CatalogProdu...p938-edge.aspx
(I wonder if you couldnt duplicate the old Colt Royal Blue by using PVD.)
I agree that rust and caustic blues are invasive oxidation and not additive like plating. Most of the chemicals I can think of used in rust bluing are pretty corrosive. Its all the micro-pits they have eaten in the steel that give it that satin look and, as you say, provide a home for oil.
Regards,