From the moment Blanche (Vivien Leigh) landed on Stanley and Stella's doorstep, she knew she was out of place. She was used to the upper society of Mississippi, and was greatly shocked to find that her sister was living in a two-room apartment in the New Orleans slums. It was the last straw after a long line of stressful occurrences in Tennessee Williams' 1947 A Streetcar Named Desire, filmed in 1951 by Elia Kazan.
Blanche believed herself to be of superior make. She had been brought up in a cultured, learned, and once wealthy family, something she reminded her sister, Stella (Kim Hunter) about, when trying to get her to leave Stanley (Marlon Brando). When referring to Mitch (Karl Malden), she defended her decision to try to marry him that as she got older, she became of greater value, that a woman of breeding and culture was of more value to a man as she got older than a young woman with none.
All the same, she was obsessed with youth. She kept the lights dimmed and would not be seen before twilight so that no one could see how old she really was. She always protested how little alcohol she drank right before dipping into another glass. She wore her old clothes from her youth, and she had a taboo interest in younger men. The tragic suicide of her lover/husband when she was still a girl, that she had unthinkingly driven him to, slightly unhinged her. She could still hear the music and the gunshot in her head at emotional moments. She wanted to bring that time back--perhaps to bring him back.
The time after that had not been pleasant either. Blanche had been forced to take care of dying relatives and deal with the loss of her home. With nothing left to hang onto, she moved to the Flamingo, where she tried to fill up her lonely nights with many men. This lifestyle certainly did not live up to her idea of youth, so when she was forced to leave town, she started afresh with her innocent act, this time with Mitch.
Mitch was her last ditch effort for a new life. It was not that she really loved him; it was that he could take care of her and would be there for her. He was not violent like Stanley, and she could probably control him better. But because of her mistreatment of Stanley and his discovery of her hypocrisy, he told Mitch all about her sordid past. Without that last shred of hope that she had found in Mitch, Blanche was trapped. She began lying about a wealthy admirer, wallowing in fantasies about younger days.
And after Stanley had raped her, her last hold onto reality was broken. She began to believe her own fantasies and lies, unable to tell the difference between the real world and the world she wanted. Her fears and wishes poured over into illusion.