What is the importance of the Blue Piano?

The blue piano is usually invoked in scenes of great passion; Williams states in the opening stage directions that it "expresses the spirit of the life" of Elysian Fields. It is indicated that this music should be most present in the parallel scenes of Stella's lustful reunion with Stanley in Scene 3 and Blanche's rape in Scene 9, as well as at the very beginning and end of the play, in the two moments that the Kowalskis share without Blanche in their lives.