I have used the brownells brand on a shotgun and it turned out well, but I am not sure this would be 100% correct for a luger. I would add to Ice's comments that to help keep flat surfaces flat, always sand with a block where you can, and to be careful not to round off corners. Go easy around stamps and other marks.
Keeping everything oil free is critical. Break cleaner is good at this and cheap, also I keep my steel wool for carding in some acetone, that way if I do contaminate things slightly they get cleaned off as I work each rust cycle. Keep an eye on the rust progress, too much will etch the metal, not enough doesn't get anything done. It should look good and "flat" to velvety but not much more. Also don't short your parts on boil time or you won't get a good conversion from red oxide to black.
Don't forget when you get bored you won't do your best work. Rust blue is something you can do but it is tedious. So if you loose focus get to a point where you can leave things and take a break for an hour or call it a day, otherwise you won't be happy with the results.
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