Dark Knight is broken.

[TouchandFeel]

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I don't disagree with what you said and agree that more could be done to utilize the Blood Gauge.
However, I would say that the way a job/class plays in a game defines it's identity and vice versa. They are intrinsically linked. DRK has a thematic identity, that's not a problem, it's the fact that it doesn't have a well defined and implemented game-play identity.

I don't feel that making DA a trait or something rolled into Darkside really helps anything nor makes the job more interesting, it just strips away the jobs main identifying game-play gimmick and makes it a passive part of the job. That seems more boring and bland to me.

Making it so that Dark Arts is a short 10s buff may help with the problem of double-weaving but it again makes it more bland by removing more active control and decision making for essentially constant, rolling mini buff/burst windows. I don't feel that really fixes the overly flat and consistent feel to DRK right now, it just changes the flavor ever so slightly.

As I stated earlier, I strongly feel that Dark Arts could be good but it needs to do more than just add potency to a bunch of stuff. There needs to be more actual choice and decision making provided from Dark Arts, not many roads that all lead to the same destination.

The shards idea was just something off-the-cuff as an idea to be provide a way for a DRK player to have the choice to actively shift their damage cadence by spreading out the cost, but then shifting and grouping the effect so that they can have a form of controlled burst to make use of things like party buffs or to adjust to things like boss jumps and the like. That and it would add an extra layer of complexity, which DRK could really use imo. By no means do I think it is a perfect idea as was presented and it could use refinement, I'm sure, but I would much rather add something like that than essentially take away Dark Arts.

EDIT:

Another idea for something like the shards that just popped into my head but utilizes Blood instead of Dark Arts.
Rework Delerium so that it works as follows:

Delirium - Lvl.66 – Ability <-- shifted to lvl.66 while Quietus and Blood Spiller are shifted to 62 and 64.
Cast: Instant – Recast:80s – Cost: variable – Range: 0y – Radius: 0y
Restores MP.
Additional Effect: While active the Blood cost to use any abilities is removed and the Blood Gauge drains by 10 every second. The effect ends when the Blood Gauge is empty.

This is very similar to a mash-up of the old Inner Release and the new Inner Release, which imo is very fun, so it could definitely use some refinement or adjustment but eh ... just something that popped into my head as a Blood fueled alternative for providing DRK some burst.

Other abilities such as Blood Weapon and Blood Price would probably need some adjustment to make up for the loss of the current Delirium.