I feel like the Darkanas aren't quite "dark" enough, if you know what I mean; the only one with a downside in any capacity is VI - Moonlight Bolero, and it's not really threatening at all. In my opinion, Darkanas would be much more interesting to use if they were like the Curse stat or the Muramasa; very powerful, but very deadly if not used properly.
Here are some ideas I came up with on my own:
XII - Crystal Cries: The special pickups also reduce Move Speed, all the way down to -100%.
XXI - Wandering the Jet Black: Taking damage reduces Max Health by 0.2.
I'm not as confident in my ideas for the remaining three, because they would still be rather easy to avoid:
I - Sapphire Mist: The player takes double damage from all sources.
VI - Moonlight Bolero: The extra boss ignores knockback and all effects, like an Atlantean with less health. The random passive chest is very likely to give Torrona's Box or Skull O' Maniac before anything else, and can give items that have been banished.
X - Hail from the Future: Honestly, I've got nothing. Maybe it has a chance to spawn falling stars, or even Cosmic Egg bosses? That's my best idea.

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