DEpends on the maker and depends on the material. Greeners old barrel browner ,he always insisterd he was a browner not a blacker ,Ernie Pickthorn ,told me that he used to "rust black" a number of small parts both for double guns the GP guns .This was because of the use of castings that would not colour well or on which the colour faded to a plum colour quite quickly.On new guns the rule of thumb would be that all parts that were not colour hardened would be blacked . Some makers sent there guns out all blacked ,Cogswell and Harrison is a good example .
