Having never owned a Bushmaster, or any Line SI product, is it possible the "blue" look is from the same stuff they "blue" guns with?
Nope.. gun blueing is slightly different.... Bluing (steel) - Wikipedia, the free encyclopedia
Anodizing aluminum Anodizing - Wikipedia, the free encyclopedia
Both are an oxidation process... but that's about where the similarity ends.
The blue/black of old paintguns was partially from the color of ink and also the aging of the coating (sun exposure, wear, etc)