Spellbooks from another branch of the same line - upgrade from 2nd level Black Mage to 3rd level Dark Mage. There is not so much difference between variations, but those who had "shadow" on their 2nd level, may take "shadow dance" on 3rd.
generally, black mages are antipodes of loyalist white mages - they are mostly supporting units with abilities to slow opponents, or to heal allies, or to change times of Day around them (of course, they did all of this unlike white mages but in their own ''dark'' manner). They also able to inflict arcane damage, but it isn't very high and there is only one main offensive spell (unlike red or white mages who have a rich selection of such spells). that's funny but quite logically - black mages able to learn attack with backstab special (like some kind of assassins).
