Part of the anticipation leading up to Black Swan's release, at least in the fashion world, had to do with the fact that Rodarte had designed the ballet costumes. The Mulleavy sisters created 40 costumes for the Darren Aronofsky thriller, including outfitting the entire ballet corps. So it follows that Rodarte has been the bold faced name associated with Black Swan's beautifully twisted costumes.
But with awards season in full swing, you won't see Kate and Laura Mulleavy's names next to the nominations for Best Costume Design (Black Swan has already received a best costume nom from the Critics Choice Awards and will likely get an Oscar nomination). Amy Westcott will take the credit, as she received the "front credit" as costume designer for the film while the Mulleavy sisters only receive a backend credit.
In The Hollywood Reporter, Merle Ginsberg notes there's "a long history of fashion designers creating costumes for stars above and beyond what a film's costume designer does--and not getting the credit they crave." The most famous example of this, Ginsberg points out, is Edith Head winning the Oscar for Sabrina when Givenchy outfitted Hepburn for the film.
In the case of Rodarte for Black Swan, a source told THR that the Mulleavy's were "naive about movies," meaning they didn't negotiate credits in their initial deal, and weren't members of the the Costume Design Guild when they worked on the film (they are now). Fox Searchlight reportedly wanted to submit Westcott and the Mulleavy sisters for Oscar consideration, but guild rules dictate that only the front credit designer is eligible.
It's a mistake the Mulleavy's won't make twice. According to THR, "Now the sisters are repped by WME and seeking movie projects outside of producing the chic Rodarte line--and this time they want their names up front."