This could be good, if it wasn't so wrong from what a tank actually needs. There is no suggestion of a higher HP pool to be able to soak big hits like warriors, or a flat damage reduction to be able to add mitigation to tank crunches. Essentially, it's a Warrior with a Paladin's HP, but reactive healing instead of a healing % increase or a proactive damage reduction. This would be great, if we weren't playing a game that time after time has bosses with damage that will one shot a tank for 150% to 200% of their HP. A simple "pop a cooldown" isn't going to fix damage from a skill that requires not only your cooldown, but also shields from your healers to survive. There's more problems too when you take into account interactions with skills from other classes. For instance, damage that is done to a healer's shield is not damage the tank receives, and thus can't trigger this skill's effect. That means the mitigation is far less, since the shield doesn't receive a boost in potency like Adloquium gets from Defiance, and doesn't receive a boost to durability like with Shield Oath. This tank stance effectively causes less benefit to very important parts of fights, particularly for tank crunches. It's a good idea, but ultimately flawed in its capabilities.
So many people are trying to suggest skills within the "theme" of the Dark Knight. The theme they believe a Dark Knight must fall into is almost always taking more damage in some way to do something. Square has already stated that they were not going to do that, so why people continue to beat that dead horse shows just how idiotic they are. The theme for Dark Knight here is not an HP soaker, like Warrior, with a high HP pool to take more damage to generate skills or sacrifice HP to do so. It isn't a turtle tank like Paladin where they have blocking and naturally high mitigation. No, what was attempted was a Parry magical tank, which frankly fell flat on it face. If you're going to make suggestions for changes, at least keep them within the ball park of what was intended so that Square might actually incorporate them into what they have already created instead of trying to steer the class away from what was really intended.
What needs to happen are small changes to the skills already supplied to us for more intelligent use. Take Darkside... no tank has a toggle like this that just stacks with their tanking buff to increase damage. What was the point here? Why would you stack a DPS buff after a tanking buff that reduces the effectiveness of the DPS buff by 20%. I say this to remind everyone that the percentages don't just cancel each other out. No, your tanking toggle is calculated first. That 20% damage down sets the base damage to be effected by Darkside when they are both activated. So it isn't a situation where you only have 5% reduced damage while both are up, no, it's actually 8% reduced damage. So what was the point here in making Darkside able to be used with Grit when the effect of Grit makes the benefit of Darkside only a 12% increase instead of its full 15%? What this is is clearly a sign of bad planning on a class. Somewhere along the lines in development, someone in all likelihood suggested for a DPS stance, just like how Paladins and Warriors have their own DPS stances, however, what happened was wires likely became crossed, and what was created was a single target buff that acted like a Bard song but wasn't restricted from being used with Grit. Guaranteed that someone somewhere, with likely more intelligence than their superiors, probably said that this skill should be a DPS stance for Dark Knight, and it should be unable to be used with Grit, and that it should just cost MP to switch between them, not that it should drain your MP. So this is my first suggestion. Make Darkside what it was probably intended to be, a DPS toggle for off tanking and remove its MP hindrance. It shouldn't drain MP, and it shouldn't make it where things like Mage's Ballad can't recover your MP.
My next changes are to Blood Price and Blood Weapon. These skills are funny really. Their cooldowns are exactly the same. Their durations are exactly the same. Their trigger effects offer the same kind of benefits (MP recovery) yet ONLY Blood Weapon is restricted to Darkside (more evidence that Darkside should have been an OT DPS stance). So here are my changes in combination to my changes to Darkside. Restrict Blood Price to only being used when under Grit. Now, with more restrictions on these skills to being only accessed when under the proper stance, there are some changes I would make to them both to make them a little bit stronger as well as more unique to the skills' names with Blood. First, on Blood Price, change the effect to be instead of returning a flat amount of MP based on your level, absorb 10% of all damage taken as MP. Essentially, this would be like a weakened form of Rampart, only instead of just mitigating damage, that mitigated damage is absorbed as MP. This leads me into my suggested changes on Blood Weapon. Currently, it returns a flat amount of MP based on your level for each physical attack you land, and increases your attack speed by 10% for 15 seconds. My issue. If your attack speed is 2.5 seconds, and you reduce it to 2.25 seconds, you have to perform 10 attacks to land an additional hit during this skill's up time. Further, no matter what your attack speed is, this is always the case. In 15 seconds, even at an instant attack speed with a 2 second recast, there is absolutely no way to perform 10 global cooldowns (20 seconds worth) which is the requirement to get another attack in with this buff. Effectively, this skill's attack speed buff is useless. So this is why I believe it needs to be changed to a 10% damage increase for 15 seconds with an added effect of the 10% increased damage done to enemies ALSO be returned as MP, and for it to include all sources of damage, not just physical attacks. These changes would be more in the theme of the "blood" aspect seen in past Final Fantasies, like Blood Sword, which absorbed a percentage of HP when doing damage, but since we already have several skills that absorb HP when doing damage, these two skills would be better suited to restoring the resource so vital to Dark Knights.
Now, one issue many Dark Knights are facing is their TP drainage. Unlike Warriors who now have a 200TP return on a 60 second cooldown, and Paladins who now have enmity added to Shield Swipe which allows them a Global to restore a fraction of TP, Dark Knights have no TP recovery options other than just waiting or using a high MP consuming skill that provides very little utility when fighting a single target enemy. So my next suggestion is this: Skills effected by Dark Arts, when used under Dark Arts, have no TP cost. This is just one issue I have with Dark Arts. Others include that it is REQUIRED to make the most use of certain cooldowns. This is poor planning considering the high necessity for quickness when it comes to properly using cooldowns. So my next suggestion for Dark Arts is make the effects it adds to cooldowns (things that do not trigger a Global Cooldown, and things that are also on long cooldowns, and generally skills that can be used during a global cooldown) the stock effects of the skills. So, with Carve and Spit, the potency should always be 150 per hit to total 450 (and actually make it three hits... if we can make something like Barrage work for Bards, how was the ball dropped on this skill?) and recover MP with each hit. On Dark Passenger, always make the potency 250 with an added Blind effect to all targets. I'll talk about Dark Dance later, because this cooldown is all kinds of messed up. So now, you have a high MP consumption skill that triggers additional effects on ONLY skills that trigger a Global Cooldown, and removes those skills' TP cost. For Abyssal Drain, keep the MP cost. If Unleash is going to cost MP, then so is Abyssal Drain.
So I mentioned Dark Dance... WTF happened here Square? You do know how your game mechanics work, right? You put a 20% evade on a skill that raises Parry? What were you thinking? If you Evade, you don't Parry at all. This skill subtracts from its own usefulness! Why would any developer do this? It feels like a tact on piece of junk onto a cooldown that could really help the class. So, here's my suggestion. Remove the Dark Arts Effect, increase the effect of the Trait from 30% to 40%.
One last thing, remove the requirement to be in Darkside on Dark Dance, Dark Passenger, and Dark Mind.
And... I'm done. TL;DR below.
Darkside - DPS toggle Buff. Initial MP cost. No MP drain. Can not be used with Grit.
Blood Weapon - Increase Damage by 10% and absorb the 10% increased damage as MP.
Blood Price - Decrease incoming damage by 10% and absorb the 10% decreased damage as MP.
Dark Arts - Activate additional effects on certain skills. Skills effected by Dark Arts do not have a TP cost. No longer effects Carve and Spit, Dark Passenger, or Dark Dance
Dark Dance - 15% increase in Parry, 40% with Trait. No Dark Arts Effect
Dark Passenger - Linear AOE with a 250 unaspected magical damage with Blind. No Dark Arts Effect
Carve and Spit - 3 (THREE) attacks with potency 150 each. Recovers MP with each hit.