Cold Bluing In The Home Workshop

[DIYSwede]

Actually both -
1) as a nice, durable finish if its ground surface is absolutely clean, nice and scratch-free to begin with, and
2) as an oil-binding outer surface preventing rust.
The bluing isn't really that rust-preventive in itself.

Not really the way to go for cover-up of machining mistakes, scratches or rust-pitted vintage gear either.
Just my 2 cents and YMMV.