Slow Rust Blueing is often called "The King of Blues" because of its deep rich color and durability.  It's perfect for those old soft soldered doubles and high grade rifles.  The slow part comes when you apply it.  It involves preparing the metal to about 400 grit (sort of a medium polish-not dull, but not real shiny), boiling the gun in pure water,  applying the blue, waiting a day for it to rust, carefully removing the rust, boiling again, applying  another coat, waiting for it to rust, removing the rust, over and over again until the color is even and deep.  Not rocket science, but a lot of slow careful work.  You end up with a blue-black finish with a soft luster.
Many old side by side shotguns were soft soldered  together.  When they are put into modern hot blue salts the barrels come apart.  Not a good thing.  Really, the only way to refinish those guns is to slow rust blue them. 
There are a lot of formulas for slow rust blue solutions.  R. H Angier, who wrote The Book on firearm blueing in 1936, listed over 56 different mixtures.  Almost every refinisher has his own secret variation of one of these, and I do too.

                                 Double Barrels (bbl's only)            $285.00
                                 Complete Side by Side gun          $420.00
                                  Handguns (any type)                   $330.00


Here's a good example of slow rust blue.  It's a blue/black semi gloss blue.  Almost transparent.  Wears better than any other blueing, and is perfect for old doubles and high end guns.   
  There are a number of techniques to do this. Here's a couple of ways I do it.  I hot blue first, then remove some of the blueing with acid.  Then, mask it off and coat it with matte baking lacquer.  Another way is to mask it off, blast it off to achieve the gray, then spray coat with matte baking lacquer.  Average cost of a receiver is $90.
Here's before and after of a fine English Double, I started polishing at 80 grit it was so rough, then 120, 220 320 and final 400.  Then 12 applications of slow rust blue. 
This is why they call it "rust" blue.  After it rusts, it's boiled in pure water for about 40 minutes.  Then carded with fine steel wool and another coat of rusting solution is appled.  Wait a day for it to rust again and repeat again and again until you're really sick of it.