A lot of suggestions here are great in terms of quality of life at lower play (fixing scathe usage etc.) but really it seems like most people are missing the mark on what I have said.

Even when played with absolute perfect execution Black Mage does not do enough of anything to justify it's slot in any end game situation.

The solutions are VERY simple. Provide much more personal damage or provide raidwide damage utility or provide unique and valuable defensive utility. A combination of these can also be used to create a strong class, however you then risk losing identity from an iconic job, and would (in my opinion) be much harder to achieve a healthy point with.

One or more of these options needs to happen for this job to be worth bringing into a raid/trial.