In the end of Nekomonogatari (Kuro), Araragi states that Hanekawa doesn't remember any of the events of Golden Week. (This is also shown in the corresponding recap episode in Monogatari Series Second Season.) However, in the Monogatari Series Second Season, during the Tsubasa Tiger arc, Hanekawa seems to know of the existence of Black Hanekawa:
In the voice-over in the first episode, she makes some allusions to having been a cat. (I don't remember what exactly she said, but I can dig that up later if I have time.
In episode 4 she comes to the conclusion that she must have been possessed by Black Hanekawa after noticing dirt in her nails (and moreover comments that this time, there were no warning headaches).
When does Hanekawa realise that she had been possessed by Black Hanekawa? Does this have anything to do with the episode prior to the school festival (not in the sense that this was actually an episode of the anime, but in that this was an instance of her being possessed) that was alluded to once or twice?