Gifted girls are notorious for hiding their abilities in order to fit in better socially. That blank look may mean she's bored, but hiding.
Then again, it may be just the way she comes across when she's listening attentively, because that's how my DD7 comes across sometimes. I have a noted tendency to lecture endlessly if nobody stops me, and sometimes I'll notice that blank look and try to wrap things up, but when I stop, DD will say, "What else?" which is her way of saying, "Please continue."
I've come to the conclusion that what looks like zoning out for most kids is, in her case, intensely focused listening.
