It is a Advance Job so no class required but if Yoshi-P follows on his original plan for Advance Jobs player will be required to obtain 2 certain Jobs to unlock Dark Knight before they can access it.

They won't be removing Class system since they have set the lore for the classes in the game already.

I won't be surprised if Dark Knight will require Players to complete the lvl 50 Paladin Job Quest and lvl 50 Warrior Job Quest just to unlock Dark Knight.