Caswell's black oxide finish or Brownell's Oxpho-Blue are both excellent (and about equal).
Most important is the process. Clean and degrease thoroughly. Then (contrary to the name of the process) - heat the piece(s). I like to boil them with something to keep them off the bottom of the pan. Let your parts boil for a few minutes so the heat can soak through. This heats and does a final degrease / clean.
Pull them out and rub on the bluing. I apply it generously and leave it on for longer than instructed...
When you're tired of waiting, wipe it off.
Card it (buff with #0000 steel wool or a good carding wheel).
Lather, rinse, repeat as many times as you like to get the darkness and/or gloss you're going for.
As a final step I toss my parts into used motor oil after the last step. This neutralizes the compound you've been using, soaks into the metal to protect it from rust, and seems to make the color a bit darker. I usually leave it for several hours.
To reiterate: the process is most important. I'd rather do it as desscribed above with Birchwood-Casey (probably the worst of the cold blues) than have someone use Caswell's blue and follow the instructions...
You might be tempted to try some test pieces, and there's nothing wrong with that, but if you're bluing a different alloy results may not be at all representative.
Good luck!
GsT