First, you have to understand that you can have GOOD bluing, or you can have CHEAP bluing.
About 95% of a good blue job is in the polishing done before the actual bluing takes place.
To develop the "feel" "touch" and skills need to do good polishing takes years of experience, and unless the person doing is does it most every day, they don't develop the skills.
Nothing looks worse than a gun with the sharp edges rounded off, and wavy flats.
To judge gun bluing you can't go by the color. Horrible polishing jobs still come out with a dark color, and guns that have been ground away by a ham-handed polisher still have a high gloss.
You have to look at the metal UNDER the bluing.
You look for dished-out holes, rounded corners, ripples in the flats and other signs of over polishing.
Most of the small gun shops no longer do gun bluing due to EPA anti-pollution laws and regs.
This is really a good thing, since these small operations only fired up the tanks every now and then, and since they didn't do a lot of bluing, the polishing was almost always substandard.
Real professional gun bluing done RIGHT usually starts around $100 to $200.
Here's some of the top re-finishers that offer GOOD bluing:
http://www.fordsguns.com/ (Top reputation)
http://www.apwcogan.com/Greetings.htm (Also top)
http://www.mahovskysmetalife.com/ (This is the cheapest place I know of that does good work. He's a little slow, but the price and quality are good)
There are of course many other places, but it's buyer beware.