To start with this drawing I simply referred the two most common promotional pictures of this film - One for the white swan(http://www.impawards.com/2010/posters/black_swan_ver7.jpg) and second for the black swan(http://www.monsterchildren.com/assets/Uploads/_resampled/SetWidth491-Black-Swan-natalie-portman-17392128-2560-1707.jpg).
I started with the white swan first then background and ended up while making black swan. I'm happy with the picture because it gives a great contrast between the dark and light areas.
I used pencil colors to portray white swan and graphite to make background and black swan.
Hope you like it :)