Darkness is one of the most prominent elements in Conrad's famous novella. There are many ways that darkness can be interpreted. Here are two possibilities:
1. Darkness as lack of knowledge - In Heart of Darkness, the protagonist and storyteller, Marlowe, went to Africa in a fog of ignorance. He really had no idea what to expect and indeed saw it as an exploration into a "blank space," when looking at a map. The Europeans who were sent there did not know or actively ignored what was happening in Africa. Conrad illustrates this part of the "dark continent" by his descriptions of the jungles. He says, in describing his initial impressions, that "the best way I can explain it to you is by saying that for a second or two I felt as though instead of goig to the centre of a continent I were about to set off for the centre of the earth." Marlowe's comment illustrates how he didn't know what lay beyond the thick trees on the banks of the Congo river.
2. Darkness as imperialism and selfishness. Kurtz is a wonderful representative of the result of the rush for ivory. He is absolutely devoured by Africa and his greed for its ivory. When he dies in the end, he speaks of "the horror, the horror," which in some circles is interpreted as his untimely epiphany that he's been overcome by these powers. Marlowe describes Kurtz's end:
He had something to say. He said it. Since I had peeped over the edge myself, I understand better the meaning of his stare that could not see the flame of the candle but was wide enough to embrace the whole universe, piercing enough to penetrate all the hearts that beat in the darkness.
Marlowe was able to stop himself before succumbing to the darkness. Kurtz was not. Here's, he's describing Kurtz's inability to come back from the edge. The dark powers of imperialism have taken over his life and he ultimately is killed by it.