Proper bluing takes a lot of very fancy and yucky chemistry, I was thinking just the cold blue you can get in any gun shop. It comes out black, not a real rugged finish but better than bare steel.

But, having started thinking along those lines I could parkerize this stuff in a nice medium gray or charcoal color. That will very definitely keep it from rusting again and would make for an interesting contrast.

Actually that'd be fun for an entire TIG welded steel mtb frame, just find a big enough tank and parkerize the whole damn thing and forget about the finish for the rest of your life.