Patinas

Over time with use, the blade of high carbon knives will oxidize, forming a dark patina on the surface of the steel. A patina will start almost instantly when cutting acidic foods, and it's not a bad thing. While subjectively not as pretty as a polished blade, this discoloration will help to prevent further and more damaging rust. Often times with high carbon chef knives, people will force patinas for this added protection.

Citric acid, Vinegar, and instant coffee are three good ways to force a patina on a knife, causing it to require much less constant care. If desired, I can finish custom knives with a patina and send it out with one.