A key aspect of Blanche's character is her revlusion of death and what she does to avoid thinking about it and to try and combat it. A much neglected section of the play is when, in Scene 8, a woman tries to sell Blanche some flowers. Note how she responds to her:

MEXICAN WOMAN
Flores.
BLANCHE
Death—I used to sit here and she used to sit over there and death was as closer as you are… We didn’t even admit we had ever heard of it.
MEXICAN WOMAN
Flores para los muertos, flores—flores…
BLANCHE
The opposite is desire.

This actually explains a lot of Blanche's behaviour, and particularly why she indulged in "intimiacies with strangers" to such a massive extent. Blanche sees desire and death as being total opposites, and this is why she places such value on her appearance and staying beautiful. For her, to not be desired is synonymous with death, and therefore she does everything she can, including avoiding bright lights, to maintain the illusion of beauty so that she can be desired.

A post expressing Blanche's feelings at the point in the play when Stanley knows her secret and it is only a matter of time before she has to go might include some of the following thoughts, but it will need fleshing out significantly!

I took a streetcar named Desire and it led me to this place, this point, where all I have sought so desperately to keep hidden has been exposed. I have lost any chance at all of marrying Mitch, thanks to Stanley and his investigations. He judges me so harshly for my past actions, and yet he doesn't understand what a woman like me has had to face, has had to do in order to avoid giving in completely! He doesn't see how, for a woman, being alone and unloved in such a bleak world is so terrible! For me, my intimacies with strangers were only an attempt of mine to avoid death and to stay alive. For a woman, to not be desired, to not be hungered after, is the same as death, and death is something I have done everything I can to avoid. How can a girl be judged for that?