It is important to remember that Blanche is a character whose goal in life is to ignore the reality of her condition. Note the way that she has lost her wealth, she is a widow, she is a loose woman and she is a drunkard. There is nothing to recommend her except the exterior she portrays of social elitism. Everything she does is an attempt to cover up the reality of her condition, as her wardrobe of clothes which, in spite of her cheapness, give her an air of elegance.
You might want to consider the following quote from Blanche as an excellent example of how she clings on to illusion in the face of reality. Let us remember that Stanley has just shown her papers that she lost Belle Reve, the family estate, because she didn't keep up her mortgage repayments. Note how Blanche responds:
There are thousands of papers, stretching back over hundreds of years, affecting Belle Reve as, piece by piece, our improvident grandfathers and father and uncles and brothers exchanged the land for their epic fornications—to put it plainly! . . . The four-letter word deprived us of our plantation, till finally all that was left—and Stella can verify that!—was the house itself and about twenty acres of ground, including a graveyard, to which now all but Stella and I have retreated.
It is a characteristic of some mentally ill people that they are never able to accept their own responsibility for the problems they face. Here, Blanche blames her own situation on her ancestors, and in particular the way that the males in her family were not able to control their sexual desires and express them in a way that was acceptable in society. She remains blind to the fact that she is a true descendent of these people, because she too is unable to express her sexual desires in a way that is acceptable. By blaming others, she maintains her own illusion.