Some initial impressions:
For what little it matters, the "new" 60 DRK feels quite empty compared to the "old" 60 DRK, to say nothing of 50 and below. However once you have most of the Blood Gauge kit on the road to 70 it becomes pretty fun. Lot of buttons, lot of spamming Dark Arts.
Blood Gauge strikes an interesting balance with Dark Arts and MP management (simplified now that Darkside doesn't drain MP). Gauge is gained by:
10 for every Souleater (combo'd, of course).
1 per tick, per target, for Salted Earth.
3 per physical attack landed during Blood Weapon (requires trait at 66).
1 per hit taken during Blood Price (w/66 trait).
4 per tick while Blood Price is active (w/66 trait).
The tooltips for gauge gain with Salted and Blood Price are pretty misleading.
Delirium is a really nice cooldown as it gives extra duration on Blood Weapon or Blood Price (8s and 16s) as well as giving you a MP "chunk". The timer extensions generally make up a good chunk of Delirium's Blood Gauge cost as well.
Quietus is a decent AoE spender for Blood Gauge (and notably you can rack up a lot of refunded MP if you use this in a pack with Blood Weapon). Bloodspiller needs no real introduction. Both of these WS also do not break your combo. Neither does Grit or Unleash, if those were not new changes.
The Blackest Night seems like a very interesting way to gain Blood Gauge and good utility besides, especially given the low cooldown of TBN itself, however making the trade of MP for Blood probably needs to be weighed with the situation as TBN costs the same as Dark Arts and Dark Arts further enhances Bloodspiller and Quietus.
Dark Arts potency gains:
+350 - Carve and Spit
+175 - Bloodspiller (Grit)
+140 - Bloodspiller (no Grit), Syphon Strike, Souleater
+110 and Blind effect - Dark Passenger*
+50 - Quietus (AoE)
Increased enmity - Power Slash
HP Drain effect - Abyssal Drain
15%->30% magic vuln down - Dark Mind
*Passenger's MP cost is extremely high, same as Dark Arts, which makes it virtually the worst use of Dark Arts and even the base skill isn't that great now.