In my headcanon the Dark One’s curse isn’t tied to The Curse.
Long ago, Blue was in love with a mortal man. But like all mortal men, this one was dying. So she enchanted a knife that when plunged into her TL’s heart would make him immortal so they could live together forever. The only problem was that it made him evil. He became the first Dark One. Blue, in a panic, didn’t think things through and hired a mercenary to kill the Original Dark One with the enchanted dagger but instead, all magic comes with a price, and the mercenary became the next Dark One. And on and on it went. This is why Blue doesn’t think fairies should fall in love, ti ends badly. So she hardened her heart against TL and decided to play both sides of the field, working for whatever side she thought would “win” in the situtation.