Kurouchi knives retain the scaly residue left from the forging process, meaning that the residue works as coating on the carbon steel to prevent from rust.
The finish of Kurouch reduces reactivity on carbon steel knives, and give the knife a very characterful, rustic aesthetic prized by many knife enthusiasts.
Kurouchi finishes vary widely in appearance, uniformity, and durability. Rarely do pictures do these finishes justice as they have a very complex look and feel to them.
Of course you need to sharpen the edge everytime after using it even if the Kurouchi coating works for the sides of blade.