I think the Dark One power was once combined with Blue’s power, (considering that she’s considered the “original power”), and somehow got separated. Whether it was Blue trying to rid herself of dark magic, or someone else stealing it from her, I dunno. Maybe it’s the latter though, and someone stabbed Blue, and that’s how the dark magic in her got transferred into the blade. Perhaps? That doesn’t solve the question of who made the blade though.
That is a good theory about the dark magic in the Blue Fairy transferred to the dagger.