Yet Another WTF Moment in Harry Potter

Blaise Zabini, her pupils vast and her smile vacant, began talking to him at one point, and her manner was entirely that of her old self, with no awkwardness at all. Draco felt a momentary surge of optimism, but realized within seconds that she was charmed up to the gills, and was labouring under the misapprehension that they were both still fifth years. And, it soon transpired, that he was Pansy Parkinson. He briefly considered staying with her anyway, because the illusion was surprisingly tempting and having somebody prepared to talk to him made such a pleasant change -- but in the end his pride got the better of him, and he walked away.

Did I miss the point in Harry Potter where everyone thought Blaise was a girl?  I thought this was stated pretty clearly in the books...