Blanche is a complex character. On the one hand, she does have obvious faults. She is untruthful (she lies outrageously about her age, for one thing), promiscuous, drinks quite heavily and is hypocritical in the way that she pretends to be demure and chaste. She is also rather bossy; she tries to re-arrange Stella and Stanley's home to her own liking, oblivious of the fact that they might not like her interfering. However, we are given glimpses into her past which help explain why she acts the way she does. We learn that she was married very young to a boy who turned out to be homosexual and who committed suicide in despair. We learn further that she was left to cope with the death of many relatives and the loss of the family home, Belle Reve.
Blanche is essentially a very lonely person who has been cast adrift in a new, strange, raw world. Her romantic ideals and affectation of genteel aristocratic values are her way of trying to cling onto better things. She denies reality because she cannot cope. Although she does provoke other people, notably Stanley, we can also pity her as she is so lost. And Stanley's brutality towards her at the end, coupled with the fact that she is committed to a mental home by her own sister, increases our pity for her. She has had to cope with much tragedy in her life, and her character has become somewhat warped as a result, but we feel sorry for her in all that she has to endure. There is really no-one that she can rely on. Had the circumstances of her life been different, she could simply have been a sweet, charming, well-read and attractive woman; we feel for the tragic waste of her character and potential.