Why did PLD get such a buff

[Reynhart]

You have to be careful with this.

Easy.
Increase Soul Eater's basic potency to 350, and make Dark Arts only increase the leech effect to 150%. There, one less skill where you would use Dark Arts as an OT. It would probably be a little DPS loss, since you use DA on the majority of Soul Eater, but it wouldn't really be a big deal.
It's the same for Dark Dance and Dark Mind, wich are not very useful when you're not MT.

This way, you could have DA+Blood Weapon refill HP or TP, since you'd need less MP back.

This way, wether you're MT or OT, you wouldn't spend Dark Arts on the same thing, and you could have a very different feel and a distinct utility.