Dark knight is my favorite tank and it's a shame that it's in the state that it is.
When I first started playing the game I actually wanted to go dark knight but was unable to for obvious reasons, so I picked up paladin instead.
By the time I got to heavensward as a sprout it was the first thing I did, unlock that job and I really enjoyed it.
It's been one of the jobs I've always enjoyed in the franchise ever since I could be one in FFIII

One suggestion I would have is more in line with completely revamping it to resemble more what the job is based off of, a job that uses its own hp to deal major damage.
I know that's what the blood gauge is meant to represent, but I think having high risk/high reward would be a lot of fun, and it could have more self sustain to keep itself alive in a pinch
However I don't know if it would really work in an mmo setting, as cool as the idea sounds to me in my head

I think merging tbn and oblation would be a really good idea especially if the mp cost was either reduced or went away entirely
It could maybe be bumped up to 15% damage reduction on top of the shield, but maybe that'd be too strong
I mention the increase because other tanks like paladin and gunbreaker have around that for their 25 second cooldown, on top of the other bonuses they provide