Symbolism, Imagery, Allegory
Put away your yin-yang posters: in Heart of Darkness, light doesn't necessarily symbolize pure goodness or pure enlightenment. In fact, Conrad's vision is so dark that we're not even sure he fully trusts light. As Marlow says, "sunlight can be made to lie, too" (3.50).
Over and over, we see light giving way to darkness: the sun sets, sane people go crazy, and the white ivory introduces a brutal trade. And over and over, we see black and white merging: Brussels as a "whited sepulcher" (1.21); the ivory deep in the black jungle; the white-capped woman knitting with black wool (1.24), the Intended as a "pale head" dressed "all in black" (3.53).
And then, in case you weren't quite confused enough, everything gets more complicated: Marlow compares white men to black men, and concludes (potentially) that these men are all the same. That doesn't sound so confusing? Well, consider what happens when his steamboat is stuck in the fog: he says that the fog is so thick that they can't tell up from down. Without understanding differences—like the difference between black and white, or up and down—you can't tell anything at all. There's no meaning. Doesn't that sound pretty horrific?