The common saying is girls from North Korea look better and guys from South Korea look better (This could be mal-nutrition though since average height of NK guys is much shorter). Since Korean penisula is fairly small so people actually get mixed up. One of my best friends' grandfather came to China from Busan to escape gameble debt. (My friend also got addicted to gambling for years in Japan, he told us like grandfather, like grandson.
) In addition, Incheon is pretty closed to Weihai (shangdong) so a lot of Shandong native went to Korea to settle as well. For instance, the ancestor of former Korean president 卢泰愚(Roh Tae-woo, 노태우)was from Shandong. Years ago I went to dinner in a small Korean restaurant close to Grand Canyon, the owner told me his ancestor was from Shangdong as well. So I don't think there's such thing as pure blood Korean, just like there's no pure Han either.