While it would be a good idea to give Black Mages some offensive raid utility, I don't think it's going to happen. Even in future expansions I'd expect at most the MNK treatment, minor offensive raid utility that no one cares about because it's such a minor increase.
If this proves to be true, there's multiple ways to close the gap.
By buffing BLM damage through pure damage AND throwing them a bone during downtime, give them a way to pause the countdown on their Astral Fire/Ice duration by using Scathe to stop/increase it for a # of seconds and a meditation-esque ability that does the same thing but doesn't require a target. This goes with the Time/Black Mage theme they seem to be going for.

Another thing to do ALSO is to nerf other current raid utility. Battle Litany, Disembowel, Trick Attacks, give Shadewalker to caster cross-role and Smokescreen to melee or range cross-role.
Buff the damage of those that lose their offensive utility by something near what they lost personally from it, not what the whole raid loses.