The Definitive Dark Knight Guide

[Allyrion]

C&S unbuffed rewards half the MP needed for a Dark Arts. C&S has a 60 second cooldown. It'll take 2 C&Ss (1 or 2min) for the MP return to provide a 'free' Dark Arts that you otherwise wouldn't have had access to - and you're sacrificing 700 potency to get it.

Assuming you use your Dark Arts on SE instead of just not using Dark Arts at all, you won't just be losing 700 potency.
You'd be losing the difference of SE's and C&S's gains, which is 210. Multiply by that by 2 to make up DA's cost.

So you'd lose 420 potency for your free Dark Arts (and SE's self healing), rather than 700.