Real stainless cannot be blued. Blueing is an oxidizing process just like rusting. The acid bath method probably destroys the surface cromium and cromium oxide (which makes stainless stainless) from the surface of the barrel so that you can actually blue the darn thing.
Its kind of stupid to do though. If you destroy the stainless qualities of the exterior of a piece of stainless steel you have basically destroyed its non-corrosive properties too. Stainless is non-corrosive because of the oxide layer that naturally forms on the exterior. If you destroy that layer, it will rust like anything else. It also won't form a new oxide layer unless it is exposed to air, so if the blued surface rusts through it will rust right through the stainless section of the part too.
This is why you should never use the same tools to cut carbon and stainless by the way. Flecks of carbon steel will adhere to the stainless and compromise the oxide layer. This will create places that the rust can go right through into the stainless with nothing to stop it.
If you have to have a dark stainless gun, I suggest one of the non-metallic spray on coatings. They won't give rust a place to get in, but they will cut down glare and make the gun look more business like.
That said, I've heard that Ruger's "blue" revolvers are actually stainless with some sort of surface finish to make them look blued. Thats just a rumor I've heard though and I don't have anything to back it up.