Haven't used it on a Luger but I have used Phil Pilkington's Rust Blue solution on other firearms - a S&W Mod 10, a Colt 1903, a Winchester 94 and several others.
The color didn't come out dark for me - color shade depends on humidity, metal composition, ambient temp and number of times applied - but rather a pleasant antique looking blue gray. Here are a couple of photos of a rusty pitted Colt that I "restored". The pics give a pretty true idea of the color achieved.
Hope this helps. The blueing solution is available from Brownells.
Charlie