If we were to compare Blanche to an animal, a delicate kitten in need would seem quite right. And indeed, she tries to make clear she needs people to take care of her.
Thornton Wilder had told Williams that Blanche is too complex a character for the stage. Williams replied: "But Thornton... People are complex."
So, as fragile as Blanche may be, she's not only that. In this scene, she is reminded of the desire that forced her to leave Oriel in the form of a young boy. At first, she's not sure how to react because we're all a little slow when we have just woken up. But awake she now is and after a few moments we see this kitty transforming into a panther and employing all her tricks to capture the little deer, though she soon realises she can't really do much with it..him.
If Blanche were a recovering alcoholic who, in the beginning of the film is sober and trying to stay that way, this shot captures the moment she is presented with a glass of wine. Or, as she might say: The tarantula has just spotted a fly.