Duracoat will bite into certain finishes. Anodizing, and Parkerizing do not require removal.
I *think* you can apply duracoat over bluing as well, but you may have to "rough up" the finish a little to give it something to bite to.
Call Lauer Weaponry to find out for sure. They'll know.
http://www.lauerweaponry.com/duracoat_faqs.cfm