They definitely knew it was offensive back then, this particular depiction of black and brown people originates from post slavery Jim Crow era racism there is no concieveable way they would not have known. The whole race isn't innocent, they knew and they have always known. There are black/brown people in asia and there always have been (India for random example), even east asia (indigenous koreans) which is what I think you're referring to. Antiblackness and colorism aren't exclusive to the west.
In some animes or mangas they never saw a black person in the really olden days thats bc of colonization that white people made the ugly representation of black people and Asian people just carriednit on bc most asian people back then have never done research on a black persons looks it was still racist and was not right but they had an excuse just that the excuse was never okay.. now that they have seen a black person in their life now they have no excuse for this..
I don't know if that character is supposed to be black or tanned but bruh . I know this a Korean comic but wow....(⊙…⊙ )