Dark Knight DD or Tank

[Fantasia]

Dark knight was only a DD in XI, right?
As far I remember it was just a heavy-armour wearing melee that had access to black magic in offline games.

In Tactics it had the largest HP pool, but was one of the strongest jobs because it tore apart its own HP to deal more damage and could steal HP/MP from other mobs too. I can't really imagine a Dark Knight being a proper tank... only thing that comes to mind was Cecil in the start of IV. Neither of those had any black magic access.