Even with having Delerium and/or Blood Weapon contribute to Darkside upkeep, it still leaves TBN->Dark Arts as the primary method of upkeep which is inherently problematic.
Buffs and their application need to be proactive and not reactive so that they stay squarely within the control of the player and are not at the whim of the content. TBN, and therefore Dark Arts, are reactive.
It's really best to just avoid tying Darkside's upkeep to Dark Arts.

If you want to make Darkside upkeep more interactive there are a myriad of other ways to do that which are based around proactive abilities, for example just off the top of my head ...
Edge/Shadow only add 10s to Darkside's time and now Blood Weapon adds 10s to Darkside's time per Weaponskill or Spell. Now Edge and Flood are more about keeping Darkside's time from draining too much until the next Blood Weapon where you can really fill it back up, plus some extra wiggle room for fight mechanics/downtime.
Not saying this is what should be done, just an example.