Using gun bluing is tricky and can get expensive. The trick is what to do after you've applied it. The phosphate process uses oil as a final surface treatment and I don't see why it can't be used after gun blue. I have a couple of black powder revolvers that I made from kits years back and the bluing is still rust free but they were not subjected to moisture, are cleaned regularly and oiled. Parts that are heat treated should be darker in shade, almost black, than a natural finish. That would apply to the forged arms and to the accelerator bar (springs, tie rod ends, spindles too). I use the metal blackening concentrate from Eastwood to get a dark shade prior to the phosphating. For a more natural looking steel, a shortened dip in a phosphate bath, two to three minutes, with a lower amount of the phosphate concentrate followed by steel wool, single 0, then oil it (WD-40).
Jim