Personally, I think it's an excellent set designation for the class, and I'm going to take my opinions on that designation from a spin-off Final Fantasy, Bravely Default.
There won't be any spoilers for the 3DS title here, but I will say that Dark Knight is a Job the player can acquire later on, that's it, nothing more.
As for the Job itself in the game, I found it to be the best iteration for it that I've used or seen yet. The Job's main trait was an ability called Adversity. For each increment of damage that the character would take in percentages of 25% I believe in relation to your max HP, your four primary attack and defense stats would increase. So Phys Defense, Phys Attack, and Mag Attack and Defense. They would cap at 150% of their start, and make it a great tank class in the game, if it wasn't designated as more of a DPS of sorts in the game itself.
What made it so excellent is that I could easily make it into a tank in the party just by shifting around gear, and making him two-hand a sword which would give him great offensive capabilities. With Adversity boosting it's stats the more damage it took, it propelled it's abilities means to do damage even higher. You couple that with another Sub-Job that lets it drain stats, and it could become a powerhouse, only having difficulty with enemies resistant or immune to dark damage.
I honestly see DRK in XIV being used in a similar manner. The more the tank is getting pounded on, the stronger it's stats become, and I believe through only my own personal conjecture, that it will be able to rival WAR perfectly in terms of damage output or to even exceed it. This is only my personal opinion as of right now, just because I think Bravely Default's DRK was the most absolute fun I've had with the Job's iteration yet, dat Rage attack.
But let's hear everyone else's thoughts, I've been absent from XIV for over a month and thought I would make this thread for the discussion, hopefully I'm not just clogging the boards with another repeat thread.