T. O'Heir said:
Case hardening isn't decorative. Case hardened colouring is though. Goes on just like cold bluing. Comes from these guys.
https://steelfxpatinas.com/
Case hardening is not decorative -- correct. And
color case hardening is not primarily decorative, either, it's just the effect/result of uneven heat in the case hardening process. For those who find the appearance attractive, it may be thought of as decorative, but that's not the reason it's done.
The kit sold in the link is called "color case-hardening
patina for a reason -- it isn't case hardening. It's a chemical (or a series of chemicals, actually) that attempts to replicate the appearance of case hardening. Done well, it looks ... okay. Done poorly (like a poor cold blue job), it looks ... pretty bad.