Black Swan is a character who first appears in "UnmAsked". Spencer finds the sketches for Black Swan's costume in "A's" lair and Aria, Emily, and Hanna spot her at the Masquerade Ball. Her identity was unknown until it was revealed in "Birds of a Feather" that it was Melissa. Melissa was sent this costume by "A" and told to wear it to the Masquerade Ball. Before Mona disclosed to Spencer that she was "A", Spencer, Hanna, Emily, and Aria believed that the Black Swan may have been "A", however, this was not the case.
Black Swan has dark hair, fair skin, and is described by Hanna as being tall and having a size two frame which is her cause for disliking her. She is seen wearing a costume similar to that of Odile in the ballet "Swan Lake". She wears a mask, which conceals her identity.
In the episode "Birds of a Feather", it is revealed that Melissa is the Black Swan and that the night of the Masquerade Ball she received a note, blackmailing her to wear the costume to the ball. The note also said that she had to distract Jenna, which is why the Black Swan was talking to Jenna. Although it is not clear Melissa believes that Mona was the one who sent her the note.