Black Magic in the Final Fantasy franchise usually refers to offensive, damaging spells. It rarely has anything to do with "evil" spells like in Western fantasy tradition. So far there is no "evil magic" class in FFXIV. FFXIV Dark Knights use "darkness" but it's just elemental darkness; not evil magic. In FF4, Dark Knight's abilities were explicitly "evil" magic, but that isn't the case in FF14.

The next expansion is adding the Reaper job, which will let you summon a demon. That's the closest to an "evil magic" user you're going to get in FFXIV, and even then you can't perform any acts of evil in FFXIV (unlike in WoW).