The later Colt process used a heat bluing method, in which the parts were packed into small steel drums with a secret mixture of bone meal, charred leather, and other ingredients.
The drums were put into a temperature controlled oven and the parts came out that gorgeous Colt pre-war blue.
In the book "Colt Firearms, 1836 To 1940" by Haven and Belden, they give a chapter on a tour of the Colt plant in which they give a fair description of how a Colt revolver was made then. This includes a discussion of the pre-war bluing process.
Astoundingly, they describe Colt using HOT BOILING GASOLINE as a pre-cleaner.
Per an old book on the early Colt factory in the pre-Civil War era, the guns were polished to possible the finest quality finish ever done on a production revolver, then the parts were blued with the hot metal/fish oil rags technique.
Included was an early 19th Century-type pen and ink drawing of the bluing room.
Shown are two or three men gathered around what appears to be a low brick charcoal pit, rubbing parts with rags.
I'm not sure at what point Colt went to the oven blue system.
After WWII Colt changed over to the modern hot salts process.
I'm not sure linseed oil was used as an actual bluing method, but it was used as a protective coating for muzzle loading rifles and shotguns.
After the parts were browned, they would continue to rust and would quickly become rough and pitted.
So, boiled linseed oil was applied and allowed to dry into a varnish-like coating. This made the metal shine and protected the metal from further rust action