The Great Gatsby Passage Analysis

Passage: “Ah, I thought so. For it were strange indeed, and not very creditable to us white-skins, if a little of our blood mixed with the African's, should, far from improving the latter's quality, have the sad effect of pouring vitriolic acid into black broth; improving the hue, perhaps, but not the wholesomeness.”
Questions: I picked this statement because this statement would be considered racist today. However, in the context of the time period of the novel, might this attitude be perceived by northerners, sans abolitionists, as a normal assumption? As I read this and other passages which seemed racist, I wondered if Melville was intentionally being so overt and exaggerated in his description of the blacks as to “hit” his readers so there