Kurtz's black mistress serves to emphasize the extent to which Kurtz has gone native, so to speak. He's been out in Africa for so long that he's started to become more African and decidedly less European. In some respects, his mistress can be seen as a metaphor for the temptations that were thought in Conrad's time to lie in wait for the unsuspecting white man once he'd set foot in the so-called Dark Continent.
Here, in the middle of the steaming jungle, thousands of miles from home, Kurtz has been seduced by what, to the average white man, are the exotic trappings of indigenous culture. There is a sense, then, that Kurtz is somewhat less of a white man for having gone native. He's certainly presented to us as displaying the kind of savagery which the predominant racial prejudice of the time attributed to black Africans.
In portraying Kurtz like this, Conrad undoubtedly hopes to remind his reader of the fine line between Western civilization and the supposed barbarism of African culture,...
(The entire section contains 5 answers and 963 words.)