Based on that she is so strict with rules I wouldn’t be surprised if she once made a big mistake which caused a big emotional pain to her. Maybe that theory is right that she accidentally created the Dark One by trying to make her BF immortal and because she wasn’t able to safe him from the darkness, she doesn’t believe in fighting against darkness and in true love anymore.
My own headcanon about Blue is very similar to that: she fell in love with a mortal man and when she realized that someday he could die, she stabbed him in the heart with an enchanted dagger but all magic comes with a price and he became the Dark One. So I can see why she is so strict if that’s the case. But she’s done a lot of other sneaky things.
Well of course I wouldn’t be surprised if you’re right and she has evil intensions. But that doesn’t have to mean that she has to be working with/for Pan. I don’t think that he is supposed to be the main antagonist of the show. I guess he is just someone who knows a lot and want to be or control a big part of it. At least that is what I’m expecting. So I would be a little dissappointed if “SneakyFairy” would just work for/with him.
I do think PP is the big villain this season but it wouldn’t surprise me if they clued the audience into what Blue’s been doing while our character remain ignorant for another season or so.