Blanche does not intentionally deceive Mitch in everything she says and does. In some respects she is being at least as sincere with him as she is with herself. She truly believes in gentility, politeness, refinement, kindness, cultivated tastes and manners, all the amenities that go with the genteel lifestyle of the Old South. She mainly deceives Mitch in the matter of her age. She never allows him to see her in the full light. She refuses to go out with him until after sunset. She deliberately lies to him when she tells him that Stella is her older sister, when Stella is actually five years younger.
Yes, Stella is my precious little sister. I call her little in spite of the fact that she's somewhat older than I. Just slightly. Less than a year.
She also deceives him about her old-fashioned virtue, desperately holding out for marriage. In Scene Nine, Mitch shows that he has been enlightened about her real age and her unsavory past, but the following quotes are significant:
You lied to me, Blanche.
Don't say I lied to you.
Lies, lies, inside and out, all lies.
Never inside, I didn't lie in my heart . . .
This is true of Blanche's complex character. She can be both realistic and romantic, chaste and corrupt, naive and worldly wise, strong and weak. She is far too complicated for Mitch to ever understand.