Separate defensive abilites from DPS resources!

SE did the first step by removing Shield Swipe in 5.0.

For everyone who doesn't know what Shield Swipe was, here a summary: Shield Swipe was an ability off the GCD with 100/150 potency on a 15 second CD. Everytime an attack successfully blocked, SW would proc and was available for use for the next 5 seconds. Sheltron (and Bulwark [+60% block rate]) was used to proc Shield Swipe for extra damage. But that would weaken PLD's mitigation, so they had to re-think whether it was worth spending gauge on a low damage attack or save it for a high damage attack, but lose DPS due to Shield Swipe not proc-in'.

With Shield Swipe gone PLD can use their gauge purely for mitigation, just as it should be.

However, this rule doesn't apply to The Blackest Night. TBN is tied to mana, and mana is a valuable resource for our damage skills - Edge/Flood of Darkness/Shadow. The cost is just the same as your damage skills - 3000 mana (or 30% of your mana pool).
On lvl 74 DRK gets the enhanced version of Edge/Flood of Darkness in form of Edge/Flood of Shadow.
[For simple math purpose, I will calculate single target only - Edge of D/S]
So, 3000 mana is worth 350 potency below level 74. On 74 and above it's 500 potency.
In 4.x one Dark Arts gave us 140 additional potency on most skills, its cost was roughly 25% of your mana pool (2400/9480), but still same as TBN.
Due to 5.0 mana regeneration revision, let's say it is equal to 3000 mana now (and to keep it simple). If our shield didn't break, we effectively lost 140 potency.
Now in 5.0, our shield TBN got stronger (from 20% on self/10% on others to 25% on all). Thus breaking our shield got even more difficult. In addition - as stated above - our new "Dark Arts" skill got buffed to 350/500 potency.
Not only did it get harder to break the shield, but the punishment for not breaking it got harder, too.

tl;dr: DRK loses up to 500 potency if the shield doesn't break!

TBN should not cost DPS!

SE needs to separate TBN from our damage skills or change it, so we don't lose damage by using our mitigation tools.