Without a doubt Blanche must be held responsible for how she led her life, yet, her ultimate tragedy of being raped and sent to the sanatorium was not her fault.
The part of which she should be held responsible is for what she had opted to do with her life after her family lost it social and economical standing. Instead of trying to get "sugar daddies", quick cash, and getting in troublesome relationships, she could have sucked up her pride and start over more humbly. Certainly doing so would have perhaps built a different attitude in her, and would have made her a much happier, and down-to- earth person.
Instead, she was haughty, pretentious, deceiving, still a flirt (a bad one that that), and still she could not move away from the past. All this is what led Kowalski to hate her so much, and what pushed him (albeit unfairly) to mess up her chances with Mitch.
Surely, what happened at the end when Kowalski did the unthinkable to her was not her fault, but this was the end of a series of miserable moments in her life, which was already going nowhere.
In other words: She had choices which she did not consider, and instead kept walking a path towards self-destruction and the hatred of others.