The tragic flaw which brings Blanche low is her unwillingness to face reality. Blanche has had a hard life, losing her young husband to suicide, losing Belle Reve, and losing most of her family. However, Blanche's method of dealing with the pain does more harm than good, only adding to her misery. She drinks often and engages in promiscuous behavior, both coping mechanisms meant to make her forget her troubles. She fears old age and death, so she lies about her age (famously avoiding harsh light that would only reveal she is no longer a very young woman) and flirts with underage teenage boys, at one point even seducing a student.
Blanche's detachment from reality most often manifests in lies. She never tells the truth but rather "what ought to be the truth" in order to make her life seem more glamorous and romantic. Unfortunately, this undermines her attempts to repair her circumstances, as in the case of Mitch, the one man willing to marry her. Once he learns about her lies, Mitch retracts his marriage proposal and leaves her. Her brother-in-law Stanley ends up uncovering the truth about Blanche's sordid past before raping her, thereby causing her full retreat into fantasy by the end of the play.