It refers to a style of ink painting, which he was inspired by when creating the finish.
https://www.britannica.com/art/suiboku-ga
Takada-San keeps his technique a secret, but others have postulated that he covers the blades in a rag with some sort of etching agent to produce the finish. So it’s sort of like a forced patina I guess, but one that doesn’t rub off easily. It has the look of almost wrinkled denim to me. Each knife is different. Mine is pretty subtle and definitely best appreciated in person.