Another meaning of the name "Blanche" suggests what post#4 discusses: the fading Southern Belle. For, the word blanch means to remove color. Also, it denotes sickness and fear--two symptoms of Blanche DuBois.
The absence of color conjures the chapter of Herman Melville's Moby Dick in which the author discusses the negative connotations of white in which it evokes spectres, shrouds, lifelessness, etc.
White is used somewhat ironically as noted above. Clearly the real Blanche and the way she tries to present herself are completely at odds with each other, and this is a central conflict that drives the play and arguably results in the tragedy of Blanche and what happens with Stanley. It really captures the sombre mood and evokes sympathy in the audience for her character - Blanche is a character who is simply unable to escape her past and is dogged by it. Reinvention only makes the past glare more furiously.
The first post is a great explanation of the use of white as a symbol of the ideal of Southern purity. I'd go a bit further and state that white fades into ivory and then almost beige, perhaps with some stains. I think of an old wedding dress that hasn't been professionally preserved, delicate and still possessing traces of its former glory, but becoming brittle and no longer the pure white it was on the wedding day, if it even were worn at all.
Blanche is like that faded wedding dress, once a beautifully frosted Southern cupcake, but now a shadow of her former self, with that former self embellished in her mind far beyond its actuality, the way that memories of a wedding may be far more beautiful than the videotape would show.
The above post is great in the description of white as it pertains to Blanche's character. Williams needs to build up Blanche's character in this way in order to present the tension between her and Stanley. The two are opposing characters, and much of their conflict has to do with the way the two present themselves. Blanche paints herself as an innocent Southern belle, yet as mentioned above, her sexual past follows her. Stanley uses this against her to try to uncover her fraudulent ways.
Tennessee Williams' uses white as a symbolic color in several ways in his play A Streetcar Named Desire. White is typically considered to be a symbol of purity, so let us first consider the name of Blanche DuBois, a central, symbolic character. The name Blanche means "white, pure." It is a word associated with purity, innocence and beauty. Perhaps Williams is trying to communicate his character's purity and innocence through the use of her name. This is interesting when the character is exposed (figuratively and literally) as having a sexual past -- her husband, whom she loved, turned out to be a homosexual and committed suicide. After his death, she desperately tried to maintain her Southern Belle lifestyle by taking many male suitors, thereby suggesting she was sexually promiscuous. Despite this tainted past, Williams presents her as a delicate, white, pure lady who did what she had to do to preserve her life. You can look through the play to find more references to "white" -- and they will all most likely surround Blanche's character.