Black Mage doesn't need a raise to be viable. It needs to be able to contribute more to the party in the form of general utility. The reason Summoner is favored is because their utility skills aren't dependent on composition like Red Mage and they're generally more mobile than Black Mage allowing the group to not figure out mechanics around them.

We've seen that high damage alone isn't enough to make a job more viable, so perhaps they could learn from black mages in earlier games (Vivi is one I'm thinking of here in particular) and let black mages elementally charge the attacks of other party members for a benefit.

An example could be that a lightning infused weapon will increase the potency of DOTs by that player as well as have a chance to refresh the duration of the BLM lightning when an attack lands, or fire to increase weapon skill potencies.