Removing the MP cost from TBN merely makes it so that it's on par with other defensives without having to sacrifice potential damage to use it. Besides, they can always add job identity and kit synergy in other ways. They could:

A. Bring back Dark Arts in a Eukrasia form on a charge system that alters what things do, such as (if they make Oblation available at level 45) using DA on Oblation just changes it to Dark Mind, sacrificing the physical defense portion for the heavy magic defense. Or making it so using it on Unmend changes it to Abyssal Drain.

B. Make it so using Blood moves requires the Darkside buff to be up, but using a Blood move drains your Darkside buff by 15s per use, so instead of being some passive that may as well not exist, it works with your kit. Combine with removing TBN's MP cost, and our MP ends up being used to keep Darkside up instead of just used whenever possible. So a Delirium use would essentially be -45s on Darkside, so you're using 1-2 Edges in that Delirium burst to avoid losing Darkside.

C. Combine the above two.