Only director Darren Aronofsky could make a psychological assault a rapturous experience. Black Swan's attack on its main character (and the audience) is like unraveling a ball of yarn; the first pinches at the string are difficult and painful, but gradually it all becomes a fulfilling roll.
Black Swan follows Nina (Natalie Portman), a ballerina who wants to be perfect. Nina has a fragile self-confidence that extends into her dancing, so when she is cast in a role that requires her to move both delicately and with a sexual confidence, she must transform herself. She must also contend with polar opposite and rival dancer Lily (Mila Kunis), whom Nina envies, hates, and lusts after.
Nina's lifestyle only permits things that aid her dancing. She lives alone with her mother (Barbara Hershey) who grooms and commands Nina like a child. We see how tightly her mother's jowls clench Nina when we learn her mother is on a first name basis with the secretary at Nina's ballet company, and see the sickening familiarity with which she puts Nina to bed and turns on the music box. This home is Nina's "normal."
Because Nina and the audience are forced to return to this home every day, we never return to a feeling of safety. So when we see Nina scratch the ragged flesh on her back, tear off her toenail, pass herself on the street, or witness her ballet company's director (Vincent Cassel) sexually abuse her, our tension never fully dissipates. After awhile "disturbing" becomes our "normal" and when we watch Nina's reflection in the mirror move differently than Nina we simply accept it.
We also accept this warped reality because Aronofsky's frequent hand-held and behind-the-head shots make us feel closer to the character. Then in the scenes that contain intensely private moments - like the scene where Nina is masturbating - Aronofsky somehow manages to raise our tension and give us a minor reprieve with laughter.