Dark Arts are the abilities used by Necromancers, each spell proving to be a powerful ability. Spells are learned from monsters, though the Necromancer need only slay a requisite enemy.
Dark Magic is a specialized and some times advanced form of Black Magic that generally steals something from other creatures. All Dark Magic is aligned with the Dark element (except Stun, which is associated with the Lightning element).
Dark Arts (Dark Magic on the PlayStation) is the actionability of the Archaeodaemon.
Dark Magic is one of the categories of magic spells, and can be equipped and used by any job.