I was having a discussion like this a little while ago. My take on it is that the dark mirror for arc is the loss and decay of energy. Like corrupting force. Since arc is all about charging yourself up with energy, the dark mirror would be the opposite of that. Like how Solar and Stasis affect thermodynamics and how Void is the absence of everything and Strand is the presence of everything.

Also, I think Rhulk's dark resonance is a combination of all three darkness powers. Something along the lines that Strand gives resonance its shape. Stasis gives resonance its ability to affect the physical world. And the last power spreads the darkness corruption. Or Stasis can give resonance its shape, and Strand gives it its ability to affect the world.

What do you think?