In scene 2, Blanche and Stanley are going back and forth in a conversation that jumps from the lost home of Belle Reve to Stella's pregnancy to the opinion of Stanley that, in some way, Blanche could be "cheating" him and Stella of what is rightfully theirs, in terms of property.
It is here when Blanche tells Stanley that, even though she fibs and embellishes things, she has never cheated anyone. She responds,
I know I fib a good deal. After all, a woman's charm is fifty percent illusion, but when a thing is important I tell the truth, and this is the truth: I haven't cheated my sister or you or anyone else as long as I have lived.
Hence, what Blanche is telling Stanley with the specific phrase that "a woman's charm is fifty percent illusion" is that women are, or should be, in some way, enigmatic. She certainly likes to be like that by telling half-truths, playing around with words, embellishing stories, and exaggerating things. Basically, in Blanche's reality, if a woman were to give out everything she has to offer, she would no longer be interesting or keep men curious. A woman's charm, or half of it, has a lot to do with the mystery and curiosity that she inspires with her personality.