I have this from a jeweller who invited me to his yearly
He had a fine bleu covered gold ring with diamonds. I asked him how
this was done and the answer I've got was CVD-process. As far as I
understand the process, metals are covered with vaporised gas
covering the metal with a layer like drills are covered.
Like I said before, I don't have enough expertise, just passing on
the little bit of that I know of to give you a start.
Have fun and enjoy.