In short, patina or rusting is the steel on the surface reacting to the food and what is in the food. The kurouchi is basically forging scale that is left on the knife. It has already reacted when it formed the scale, so the scale doesn't have much if any carbon that can react since it already has reacted. I've also seen many Kurouchi knives come with a thin, clear lacquer, which further helps keep the surface from rusting. Like Croth said, if you grind off the kurouchi finish, the bare steel under it will rust/patina/react.
I may be off a bit (going on memory at 4am!), but I have read in various knifemaking books that when the steel is forged, heat treated, etc, the very surface of the steel has been decarburized to some extent and that the carbon is burned off. Wayne Goddard told about how the very surface of the steel should be ground away after forging because the surface is decarburized. I think they story he related was that someone had a problem with a knife not holding an edge until it got several sharpenings and got back far enough to get back into the carbon rich steel. It had something to do with the amount of forging needed to shape the knife and different steels and makers would need to remove different amounts of steel from the surface to get to the carbon rich steel.