Not really sure if this belongs here, but this is the patina from my Makoto Kurosaki "Ryusei" 240mm gyuto, stainless steel cladding over Super Blue core steel.
Despite the name of this thread (and of the steel), there's no blue going on, nor has there ever been. Instead it's all black, with more of a grey near the heel. It's a really black patina in real life.
Personally I think it looks pretty cool - I find it interesting how there are lines of "no patina" going up and down (from cutting surface to the beginning of the cladding) in certain places, like those areas just decided to not react to anything. No idea why that happened. If anyone can explain why certain areas just didn't react/patina, I'd be interested to know - they're still exposed carbon steel...
This patina is all-natural and comes from using the knife on and off since I bought it in Feb 2019 (5 years ago).