The short answer would be that Blanche blames everyone but herself for her troubles. However, she especially blames her sister, Stella, for leaving Belle Reve during its period of decline.
In the first scene of the play, Blanche openly blames Stella for leaving her to deal with the loss of their family members and Belle Reve alone. Blanche accuses Stella of only thinking of herself when she left the estate and that she alone "fought for it, bled for it, almost died for it!" This resentment comes through Blanche's comments about Stella's weight (not realizing she is pregnant) and current working-class living conditions. Stella's irritation with Blanche's outbursts only makes Blanche feel more alone and vindicated in seeing her sister as a deserter while she had to watch their parents and relatives die one by one.
Blanche is especially resentful of Stella because she married Stanley Kowalski, a man who is both of Polish stock and firmly working class. Blanche sees Stanley as beneath Stella and cannot comprehend why she would have abandoned the Southern gentility for the likes of him. All of this causes Blanche to blame Stella for her current situation, even though Blanche's own bad decisions are arguably more relevant to her current joblessness and issues with alcohol and sex.