During their talk, Blanche is in the tub singing. What does she sing about? What is the significance of the song she sings in the tub?

Blanche sings about dreams and make believe..... in a way, this is the same way she views her own reality. She only sees her dreams.

"Say, it's only a paper moon, Sailing over a cardboard sea--But it wouldn't be make-believe If you believed in me!"