This kind of psychological splitting is never permanent, however, and cannot hold. The disowned and projected part of Nina's experience -- what Jung would have described as her "shadow self" -- starts to show up as a twin, a kind of dark mirror image which she glimpses from the train, along shadowy passageways, etc.
Lily, a recent addition to the company, also appears to have two different aspects. At first, she behaves in a kindly and supportive manner toward Nina; but later, she becomes fiercely competitive and seeks both to undermine Nina's self-confidence and sabotage her performance in Swan Lake. Director Aronofsky fills the movie with dual images that convey the notion of splitting: white swan/black swan, good Lily/bad Lily, and pristinely perfect, all-in-icy- white Nina, threatened by the sudden emergence of her murderous and sexually potent shadow self.
Black Swan shows how we can be empowered by reclaiming those parts of our experience that we have split off and disowned. The only way Nina can grow the emotional range to dance the black swan is by embracing her passionate sexuality and murderous feelings of jealousy and rage. There's been some debate about which parts of the film are fantasy and which scenes depict actual events, but for me, it doesn't matter. Black Swan realistically portrays mental states and psychological processes you rarely see depicted in a film.
The script, realized through Aronofsky's deft direction and artistic vision, conveys an important message: It may be frightening and socially disruptive to express our "darkness", but when we strive to be purely "white", we end up psychologically impoverished and afraid, incapable of a truly passionate engagement in our world.