Kuro uchi is "black forge finish". Western smiths call it "brut de forge". It is the darkened steel surface from forging, and usually includes the hammer marks. Higher carbon and lower alloy make it more bold. Decarb is part of the surface, and is solid. Scale is an iron oxide that will flake off. Scale is much blacker ... and harder ... but not attached very well.The trick is to darken the surface layer of decarb.
The decarb is essentially Fe. What you want to have to make it black is FeO. When the FE is exposed to very dry air, it forms the FeO bond. This type of iron oxide is not very reactive because the iron is already tied up. If the Fe is exposed to moisture and air it will form FeO3. This is rust. The dark oxide is what is called patina when it slowly forms on a knife blade with use and exposure to acids. What kuro uchi does is form this protective "anti-rust" patina layer in the forging.
There are several ways of getting a dark "forge" finish. Cycling the steel repeatedly to around 1600F and then cooling to black will build up a dark surface layer. Etching in FC will make a black surface. Brushing steel at around 700-800F with a mixture of turpentine and beeswax will give the classic "blacksmith's" finish. Heating that finish to around 100F and baking for a while will set it really hard.
The hammer marks are an important part of keeping the finish on the knife, because the raised edges of the dimples are the only part that wears. The dark steel in the bottom gets little surface wear and stays dark. If a smooth surface is darkened, it slowly wears away with use. Any polishing or heavy cleaning may completely remove the dark oxides. There are special hammers I use to put various patterns on the upper bebel on some knives to make the kuro uchi more persistent. If the beeswax finish is also used, these depressions give it a deeper place to bond, and it really lasts a long time. Besides the hammer dimples from forging with a dog's head hammer, other patterns are from using a ball peen, a cross hatch patterned hammer head, and a random rough surface patterning hammer (Looks like a cinder block).
