In all the original specifucations, the surface finishing of most small Lee Enfield parts is referred to as "Blazing off".

This seems to be a process in which the heated components, almost always "malleable cast iron" (or a variant thereof) are quickly quenched in oil and then withdrawn to let the residual oil burn, i.e., "blaze off".

I have not tried this process personally, but it seems like it should be done outdoors, on a paved surface, away from the family residence and "man-cave".

Bigger components were "BROWNED", i.e. SLOW rusted. Somewhere along the line, someone discovered that extending the process and lightly removing the surface "red" rust, a dark blue / black surface developed.

Originally, "slow blueing" really meant SLOW! Days for letting things develop a fine red-rust "skin", then "carding" back to the desired black-ish oxide below; rinse and repeat, literally. Then in another one of those fascinating leaps of technology some clever lad invented the "steam box". Done correctly on a properly prepared and polished surface, the blue is so deep you can almost drown in it.

Entertaining "steam" blueing video here: