Well, mathematically full moon is only a infinitesimally small split of a second. When I made the comment it was about 12% waxing crescent thus 88% in the dark. And actually darker than a full moon because the earth does not light up the far side.
They want to fly by at lunar sunrise as the shadows help see depth better. Also, they have very sensitive cameras (up to 3,280,000 ISO!); the Earth photo the other day was taken at night, so you can see how they'll be able to get detail even in the dark parts