Stainless can be "blued", it just isn't as easy to do as chem blacking of other non stainless steels; or indeed if you want it black then just get it nitrided.
Do the coating after; albeit it's cheap insurance to talk to your smith about this.
As to finishes, I'd say talk to PTG to see if it'd be okay to Melonite it. If so, that's the way to go. And if you want to trick it out a bit, have it mod'ed so it looks like THIS
'Course some, including me, love the look of shiny flutes & a dark colored bolt. Perhaps even have the flutes titanium nitrided (?).
There also are a couple other recent threads about bolt coatings which may be of help. One's called To Ion-bon or Not??? And it has a link to another called Cerakotes Micro Slick?