Black Swan is a mirroring exercise (often literally so), projecting different elements of Nina Sayers's personality outward onto doubles, reflecting life with art or vice versa, and color-coding throughout to emphasize classical dualities, especially sexual. The obvious symbolism of the film's last-act penetration (with a shard of a dressing-room mirror) confirms that Black Swan is basically a game of fuck/marry/kill featuring Natalie Portman's id, ego and superego, and Aronofsky treats this Freudian hokum as a high-class horror movie, obsessing over the process of being en pointe as he once did about the process of finding a vein, and manipulating offscreen space with genre-flick verve.
This leaves room for some skepticism—Jeff Reichert effectively flays back the movie's simple structure and finds nothing underneath—but I do think there's actually a there here (beyond, I mean, the surface pleasures of overbudgeted grand guingol). As I started to say in my Top 10 list, Aronofsky's batshit ballet is grounded, I think, in a genuinely harrowing portrait of extreme female self-discipline. It helps, I think, to remember that anorexics often stop menstruating.
This is not to say that Black Swan is a movie about an eating disorder, obviously—though Nina never seems more revulsed than when her mother presents her with an unplanned dessert: presumably she'd be deviating from a strict nutrition plan, one of the many largely self-regulated structures that guides her life.
Like breakfast: earlier on, Nina and her mother are like little girls together, exaggeratedly fawning over a half grapefruit: "preeeety". A girl's best friend is her mother; both wear their hair pulled back in tight ponytails. (Dancers often get receding hairlines from their ponytails yanking out the hair at the top of their foreheads by the roots.)
And later, when she starts to get a rash on her back that may or may not be her black-swan wings coming in—a metamorphosis into a more sexually advanced state—her mother, citing Nina's history of scratching, takes out the fingernail clippers and declaws her.
So Nina is infantilized, both by her mother and the rigorous self-discipline her profession demands. But the movie seems to suggest that she's right to stay gripped so tightly: the friend who wants to take her out to cut loose at a club really does want to usurp her role; the director telling her to get more in touch with her sexuality, calls her "frigid," really does just want to feel her up. The politics here are dubious and seemingly self-contradictory—but maybe a way to look at it is that the entire narrative is subjective, not just the hallucination sequences (and it's harder to separate them as the movie goes along). In a number of early dialogue scenes with Portman, Vincent Cassel seems to be looking straight into a handheld camera during his reverse-shot close-ups—it's subtle, but I think they're p.o.v. shots. Even Aronofsky's back-of-the-head follow shots, left over from The Wrestler, carry Nina from location to location with match cuts eliding much of the space between home and work: how much of a world is there outside of what we see? Black Swan depicts an adult world of sex and disorder the way too-perfect Nina would see it. How horrifying.