About 5 or 10 minutes into the film a bunk mate of Joan Fontaine's character is trying to get her to go on a double date with her and her beau. When Joan declines, the friend replies with concern "You ain't a blue stocking?!" Joan replies "Oh I should hope not."
What does "blue stocking" mean?
It sounds like an age reference. Older women would wear dark stockings to cover up their aging legs. Light, sheer or beige stockings would allow signs of age like varicose veins to show through. However, black stockings were considered too sexy, as they had a strong association with dancers and others dressing to be alluring. So, they would opt for blue. Saying that Joan's character is not a "blue stocking" is like saying she is not a "blue hair."
A bluestocking is an intellectual woman. In the scene you reference, she is being asked if she prefers books over socializing.
shareYes, I'd agree and say that brbroose is correct in this case.
I think of a bluestocking as a brainy, intellectually-bent woman who would often be looked at in a derogatory manner for preferring educational stimulation over socializing for "mere" fun.