One of the first things we learn about Blanche is that she longs to cling to her image of an innocent, beautiful southern belle. She proudly tells Stella that "I weigh what I weighed the summer you left Belle Reve" and asks her to "turn that over-light off! I won't...
One of the first things we learn about Blanche is that she longs to cling to her image of an innocent, beautiful southern belle. She proudly tells Stella that "I weigh what I weighed the summer you left Belle Reve" and asks her to "turn that over-light off! I won't be looked at in this merciless glare!" Despite the fact that she is clearly no longer the virginal belle, Blanche clings to this identity as a way of rewriting her past; if she can position herself as the naive ingénue, she has a better chance of "snaring" a man and therefore ensuring her economic stability. Thus, she desires to be seen as young and inexperienced, and this desire stands in such stark opposition to the truth that as her lies about the past begin to be revealed, her mental state unravels.
Blanche also desires storybook romantic love. Despite the fact that Mitch is clearly her intellectual inferior, she manipulates their relationship so that she can pretend Mitch is the ideal suitor and thus enact the rituals of courtship. For example, when Mitch and Blanche return to Stella's flat after their date, Blanche says, "We are going to be very Bohemian. We are going to pretend that we are sitting in a little artists' cafe on the Left Bank in Paris!" Mitch could be any man here; his purpose is merely to function in the role Blanche has cast him in so that she can take pleasure in the fantasy of falling in love.
Lastly, Blanche desires to be open about her sexual wants while at the same time maintaining the decorum of a proper southern woman, an impossible balance to accomplish. She tries to be both women—the sexual one in her flirting with Stanley and the proper one in her relationship with Mitch. But those two identities are irreconcilable: the sexual side of her is punished when Stanley rapes her, and the proper side of her is punished when Mitch becomes disgusted by her, telling her "You're not clean enough to bring in the house with my mother."
Blanche is an incredibly complex character in that she is both victim and perpetrator. Her being taken away to a mental asylum at the play's end feels like the best possible solution: at least there she can live away from her rapist, in the world of "make-believe" she always sings about while she bathes.