While it was believed that Boy Blue had met and fallen in love with the real Red Riding Hood. Truth was, she was no more than an imposter. Most likely a sorceress who had used a spell. Though its not known what she did or what she learned while at the last Castle. She did have a brief romantic relationship, though its not known if that was real either. Given the fact that she was a spy, she could have been using Boy Blue from the very beginning.
Though Blue escaped the siege, its said "Red" stayed behind to fight and wait for Blue. Its unknown what became of her imposter.