Mike,

I've had this done and I've seen it done. Ironically, every piece I've seen was actually painted, not chemically "controlled rusted" as in conventional bluing.

Duracote and similar finishes can be had in matte to gloss varieties. No one can tell its paint if it is done correctly and it is exceedingly durable. I had an Orvis Beretta Uplander done this way with a spectacular wood stock attached and no one would ever have believed it was painted.