Banned In Barbados: The movie was temporarily banned due to the lesbian sex scene between Nina and Lily, which censors saw as "offensive sexual behavior." Public protest and a petition against the ban got it overturned and the film was eventually released with an R-rating.
The Danza: Sergio Torrado as Sergio (the dancer playing Rothbart in the Swan Lake production).
Dyeing for Your Art: Portman and Kunis lost 20 pounds each to play ballet dancers. They also both learned ballet for months prior to shooting, 5 hours a day. This phenomenon, of course, is the entire point of the movie.
In-story example. Thomas walks a dangerously thin line between sexually harassing and sexually assaulting Nina in an effort to teach her how to embody the role of the sensual Black Swan. Or maybe he's just a pervert.
Natalie Portman said that Aronofsky tried to create a rivalry between her and Mila Kunis, by keeping them apart and telling each that the other one is doing a better job.
Noticeable for those who are fans of Aronofsky's work, the bald guy in the suit on the subway is played by Stanley Herman, who played Uncle Hank from Requiem for a Dream (Aronofskysaid that it's actually the same character) and the bald guy in the suit on the subway (aka the Moustacheless Man) in π.
One of the wealthy attendees of the party is played by Mark Margolis, who Aronofsky fans will recognize as Sol from π, Mr. Rabinowitz from Requiem for a Dream, Father Avila from The Fountain, and Lenny from The Wrestler.
Romance on the Set: Natalie Portman and the film's choreographer Benjamin Millepied. They're now married and have a child together. It became Hilarious in Hindsight as Benjamin Millepied is the guy Thomas asks if he would do Nina, Natalie's character. He answers no. Portman, heavily pregnant during her thank you speech at the Golden Globes, said: "He's the best actor! It's not true! He totally wants to sleep with me!"
Star-Making Role: While both Kunis and Portman were already renowned actresses before the film came out- the latter probably more so than the former- the film has almost certainly increased both their credentials as "serious" thespians. Portman even won an Oscar for her performance. Contrast this role with others that Portman shot before it but which were released after, such as Your Highness, for a marked illustration of this effect.
What Could Have Been: Summer Glau revealed at Emerald City Comicon 2012, that she had auditioned for a supporting role in this movie, but during the pre-production stages, the character was cut from the story.