Board Thread:Character Discussion/@comment-3574574-20161227195711/@comment-25375217-20161227204039

We don't know yet, but it's been confirmed to be covered at some point in the season.

My speculation is that A) she likes to irritate Rumple, but also B) she likes to irritate the fairy community. Because of what we've seen from Blue, it's a safe bet that Black was forced to give up Rumple because of the no-love-for-fairies rule. If this is what drove her crazy and made her go evil, I think that she might be kidnapping children from their fairy godmothers in order to humiliate Blue and the other fairies as revenge for them wronging her. So when she heard that Blue was now Gideon's fairy godmother, she saw a chance she couldn't pass up: not only humiliate fairy godmothers in general but the fairy who she specifically despises, while also getting her hands on her grandson and ticking off Rumple.