Black TN
The way it was 'splained to me is the gas used during the plating process changes the color of the titanium plating.
Titanium Nitride (including Gold TN) is a ceramic not a metal but it has a lot of properties of a metal. The natural color is gold.
Black TN differs a little chemically from Gold....
Here is what Wikipedia has to say about Black TN. My highlights.
"Other commercial variants
There are several commercially-used variants of TiN that have been developed in the past decade, such as titanium carbon nitride (TiCN),
titanium aluminium nitride (TiAlN or AlTiN), and titanium aluminum carbon nitride, which may be used individually or in alternating layers with TiN.
These coatings offer similar or superior enhancements in corrosion resistance and hardness, and additional colors ranging from light gray to nearly black, to a dark
iridescent bluish-purple depending on the exact process of application. These coatings are becoming common on sporting goods, particularly
knives and
handguns, where they are used for both cosmetic and functional reasons.
[9]"