I don't see why everyone is overthinking Dark Knight Fixes its very simple.

1.) Remove the Mana Cost from TBN and make it take 20 seconds off Dark Side for each use. This would make Dark Side actually a resource that you have to monitor/be aware of or you lose DPS/Shields and it doesnt directly interfere with your Offensive Resource

2.) Make Unleash Combo abilities actual Weapon Skills and not spells so they are affected by Skill Speed, as they are currently hard locked at 2.5 speed

3.)Make Blood Gauge Abilities Heal you on use via leech in similar fashion to Paladin Magic Combo.

4.) Make Blood Gauge Abilities reduce the cooldown of Shadowbringer so you can actually use 2 in the same burst window outside your initial opener.

5.) Make Oblation provide a lifesteal effect similar to bloodbath for the duration.

And just like that. Dark Knight is completely fixed