Ok. Blue-ing is more commonly referred to as the coloring on a gun barrel than Dykem in a bottle for touching off parts.

Blueing is black oxide, yes? Yes it is. It had the name blueing long before you decided that wasn’t the correct science name.

The appearance of black oxide depends much on 1)Black oxide application can be achieved in a number of ways. But importantly 2) Surface finish. I know a black bolt looks matte black and rough. And I know a gun barrel has a nice sheen, black and shiny. Because the base metal is different, the sealant applied may be wax can lacquers vs gun oil, and the process is different ends.

There are quite a few ways to achieve black oxide, the original was sizzling a rag saturated with cow’s urine onto hot steel. Now there are other ways... over the counter and also from commercial plating/costing companies.

Someone brought up blueing on reddit, then you taught them that was just an ink for machinists to scribe part layouts or touch off of lathe parts. They really thought they learned something... but they actually were right that blueing is a conversion coating used to add color and more properties to steel, perhaps steel with chromium, too.

How is my trade too narrow? What is my trade? For your not knowing a trade name, blueing, as being analog to Black Oxide and so you wanted to teach the world about Dykem as proof of their lack of knowledge about chemical or metallurgical processes? What is your trade?