Okay, so admittedly my main is a DPS and not a Tank, but I really don't think Tanking classes should have a charge. My experience playing as a WAR is that face-pulling is not the way to go, since you could accidentally aggro more monsters than you had intended, leading to a party wipe. As such, I'd recommend making Iai Strike a melee attack. Instead, you could either make Shockwave a ranged pulling attack, or you could replace it or Iai Strike with Wind Slash (a move used by Paravirs, a Samurai-like job from FFTA2) for use as a ranged puller.

Also, Samurai lacks any sort of "Panic Button", like Marauder's Holmgang or Paladin's Sacred Ground. I know that Knight is intended to be a DPS role, but in that case, you should remove one of Samurai's offensive moves and replace it with something similar. Just as an example, but how about a move that both greatly lowers damage taken and gives you a potent regen effect? The two effects would have to be balanced such that combined they would make you effectively invincible while active while still being unique when compared to Holmgang and Sacred Ground.

Dark Knight looks good overall, though I do believe it should have native access to something like Drain, so it isn't reliant on a Second Wind variant and a cross-class skill to keep itself going without a healer. But that would mean having to decide between Darkness, Soul Eater, and Scourge to replace, and all three of them look so cool...

Anyway, those are my thoughts on this right now. You putting your own thoughts on a Samurai job and class has made me want to go back and work on my own idea again. It uses an alternate resource system similar to Warrior's Wrath, which I've kept messing with trying to get it to look good without just being a renamed Wrath...