This harmful trait is Blanche's inability to adapt to her surroundings. This is seen by noting a play on words used by Williams. Blanche cannot adapt to her surroundings, but instead tries to change them. By using the word dainty in both places Williams shows us how Blanche tries to change her surrounding to match her, instead of adapting to them. This will not work with Stanley.
Blanche deceives everyone for a good portion of the play.
However, Stanley is continually trying to find her true history. I try to give that to people. I misrepresent things to them. I don't tell the truth, I tell what ought to be the truth. This difference of philosophy creates much tension between the two.