After a few weeks without posting a blog, tonight we received a very good question from a client and we found that the answer might be pertinent to many so decide to write a little post on Christmas day about the black blade finishes, also know as Kurouchi.
For starters, Kurouchi is a traditional steel finish often used by blacksmiths which translates into "Black Forge". The finish is an unpolished layer of the steel during the heat treatment that gives the knife a very characterful, rustic aesthetic as it retain the scaly residue left from the forging process.
This finish can and will eventually wear off and can leave marks mostly visible on light cloths and even certain foods. Most commonly, cheese is an ingredient that is light in color and will tends to display Kurouchi marks once you are done cutting but is not toxic so don't be afraid.
The speed or wear will depend on your use, type of ingredients you cut and your maintenance habits, example; rubbing the blade aggressively while washing the knife or using it on acid ingredients such as lemons will affect the Kurouchi faster.
Leaving it short, this is a quick crash course on Kurouchi and I hope it gives you enough insight, however there are tons of other articles and videos available online.