Well the basis of this thread is pretty simple folks: the OP is asking if he can tank the same way he does as his Paladin, which seems to be single target focus plus Flash spam to keep the others on, with his Dark Knight.
The answer to that is pretty simple: Yes. You can. Simply do your threat attack over and over, use Unleash like flash, ensure you use Blood Price to keep your mana up, and by all means, have at it.
Now, the proper question that you should have asked, OP, is the following: Is it the most efficient way to tank?
And safe to say, that's a resounding no.
Simply put: Even with Darkside up, unless you colossally fail at your rotations and keeping your mana up, Darkside mana use should not be an issue. At all. You can easily tank with Darkside up and still only do your basic rotation, albeit you're kicking your mana rotation out the window which will make things more complicated.
Let's go into detail here:
It is entirely feasible for you to go in like this:
Run in, throw your ball of hate and darkness (Unmend), Unleash to your heart's content, Blood Price, and do your basic rotation of Hard Slash, Spinning Slash, Power Slash.
You keep that on one target, and you'll be king of keeping threat on that one target. You'll even hold your adds fine.
Ultimately? The idea is to actually reduce the amount of damage you will take in order for your healers to not have to heal you as much and keep you up and going and their own mana pull. This means, as a Dark Knight, due to the fact you have far less mitigation than your Paladin counterpart, that you want to kill things faster. And that's where it becomes a bit different as to how you want to play.
What's missing from the above rotation? Siphon Strike. This is your mana regenerating ability. Without this, you're going to run into mana issues if you keep using your other attacks. So then, you are needing to change your tactics in fighting:
Run in, throw your ball of hate and darkness (Unmend), Unleash to your heart's content, Blood Price, and do your basic rotation of Hard Slash, Spinning Slash, Power Slash, THEN do Hard Slash, Siphon Slash and Souleater combo.
This allows you to not only steal health, but regen mana as well, and keep Darkside problems to a bare minimum. This also allows you to get mana back to activate your Unleash skill you want to use so much initially. Without this rotation, you're going to run out of mana quickly during your first rotation and lose out on damage as well as some defensive cooldowns that are only present during Darkside.
Now, with that in mind, yes, your job is to mitigate damage. So now, you can throw Souleater out of the window and decide that you want to hit the casting mobs with Delirium instead. This removes the healing component of your rotation but adds a 24 second debuff on the boss which reduces the INT of an enemy by 10%.
So, now we're at a point where our rotation is far more complex.
Run in, throw your ball of hate and darkness (Unmend), Unleash to your heart's content, Blood Price, and do your basic rotation of Hard Slash, Spinning Slash, Power Slash, THEN Hard Slash, Siphon Strike, Delirium, THEN Hard Slash, Siphon Strike, Souleater.
Phew. Long rotation, right? Well now, two out of your three rotations are pretty much your DPS rotation. At some point, you have to wonder: Why would I go and do my threat rotation when the two other rotations seemingly do enough threat on my targets, and I'm still attacking the main target and keeping it on him, but he's dying faster? Well now you see how some tanks just forego that rotation after maybe one application on a boss:
So you're back to only doing HS, (Hard Slash), SS (Siphon Strike), SE (Souleater) when having to reapply Delirium on every third rotation.
At this point, you realize: Well damn. I'm only doing a DPS rotation. At this point, what could help out with threat? Why yes, more damage!
And that's where Darkside comes all the way back to play: The more DPS you throw out, the more damage you are throwing out for threat and the less need that you have to do your threat attack, which does not give you mana in order to use Unleash as often as you want. Everything is intertwined into the logic of how to play a Dark Knight.
I hope this clarified what people are trying to explain to you, OP.