Blanche is in many ways a wounded soul. She represents the epitomy of a happily ever after becoming the nightmare the next day. She is also a victim of society. She's a 'fallen woman' as far as society is concerned. She is emotionally damaged, which is why she literally and symbollically avoids the light.
You could also argue that she represents the old decaying South as she refuses to adapt to living in the post-war America like Stella did. Instead she carries the old traits with her and inevitably she id killed as she cannot survuve in the chaotic and moden world. So Williams uses Blanche as a symbol of those destroyed if unable to adapt. For example even her own sister Stella leaves her at the end of the play, refusing to accept that Stanley has raped her sister. Therfore as shechoses to abide by her abusive husband over Blanche implies again how you are doomed to be destroy. So overall Blanche represents the old South that is incapable of adapting to the modern world and hus is destroyed.