Blue
Blue turns up in 2 fields: heat treating, adhesives. Here is what it means to the people in each one.
Blue in heat treating
A chemical surface treatment that passivates steel to improve rust resistance.
Bluing creates a thin layer of black iron oxide (magnetite) on the surface of steel parts. This controlled oxidation passivates the metal to protect against rust and provides a dark cosmetic finish. It is typically done in hot alkaline salt baths and requires regular oiling to maintain its protective qualities.
"We need to blue these receiver blanks before they go to assembly."
Blue in adhesives
Shop shorthand for medium-strength threadlocker, the removable kind; red means the permanent kind.
Mechanics and assemblers name threadlockers by color. Blue is the medium-strength grade that holds against vibration but lets a fastener come out with ordinary hand tools. Red is high strength and usually needs a torch or heat gun to release. Saying a bolt got red when it should have got blue is a common complaint in maintenance shops.
"Put blue on the guard bolts, last guy used red and I had to torch every one of them."