Jajang-myun is indeed Chinese. It is a delicious noodly dish, but it is NOT Korean. Perhaps people think this because it is not usually served in Chinese restaurants (on the contrary, it can be found in Korean restaurants) and because its taste does not seem to fit in with the whole Chinese takeout scheme.
However, if you visit a Chinese restaurant in Seoul, you will be able to order it. Alas, those Chinese restaurants in America don't have this dish. At least, there are none that I am aware of. And that, my friends, is a crying shame.