carguychris is correct the the term "bluing" refers to a process, but it also refers to the resulting color -- which, as noted, varies based on the chemicals used and the underlying steel to which they are applied.
Bluing is a controlled "rusting" process and the resulting induced rust forms a hard shell that resists further corrosion with modest care.
Guns can also be "browned," which is an older process used before bluing became popular. Most of the flintlock and muskets you'll see have been browned, including modern-day recreations, if they are built in the traditional manner. It is a similar process of changing the surface of the metal to resist further corrosion. Very old "blued" guns often take on a "brown" look as they age.
Many military weapons are "parkerized," which is a manganese phosphate finish. It looks like a poorly-done blued or browned metal finish, and is not at all glossy. That resulting finish is more porous and will retain some oil to help fight rust. Not as pretty, but arguably more care-free.