Darren Aronofsky explains the advantages of having a beautiful star portray 'uncertain ground and darkness.'By Josh Wigler, with reporting by Josh Horowitz Natalie Portman in "Black Swan" Photo: Fox Searchlight Director Darren Aronofsky's "Black Swan" doesn't hit theaters until December, but many of the moviegoers who were lucky enough to see the film on the festival circuit are already predicting Oscar nominations for lead actress Natalie Portman.