Black Mage has been the job I have stuck with since beta, all through ARR, HW and now SB. I have played it in solo play, casual raid teams, world prog raid teams and even tried bringing it to speed kill teams.
Since ARR, BLM has kept very true to its identity. It stands still and it does damage. It has been given various defensive utilities over it's lifespan but these are minor additions which over time have become completely insignificant.
To get a full understanding of the current state of Black Mage as a functioning job in Stormblood, we need to understand how the game has changed since ARR when I would last say Black Mage was truly in a viable and desired spot.
1. Raid buffs.
This is probably the single most detrimental contributor to my enjoyment of Black Mage, as well as it's viability in end game content. Raid buffs such as trick attack, hypercharge, and foe requiem add strong burst windows to your teams damage. When used correctly, these allow for huge damage spikes which can be capitalized on through smart use of cooldowns and manipulating timers. It creates a layer of complexity and enjoyment when running the same content in the months we have between each raid cycle.
However. Black Mage has ZERO offensive utility. As a job it does not provide to any other team member in a positive way, in fact for many teams who are not at a higher level of play, bringing a Black Mage will cause the team to overcompensate when allowing for the turret class and this can be a hindrance to the teams overall damage.
On top of their lack of offensive utility, they are not able to consistently capitalize on these burst windows provided by other raid members. Due to the RNG nature of the BLM rotation, you cannot reliably plan which GCDs will line up with your teams burst, creating a huge disparity between getting your powerful spells within buffs and getting your weaker phase inside the same window.
2. The powercreep of ranged physical DPS.
Back in ARR, bard was very powerful. It had very high personal DPS while it provided raid dps buffs through foe requiem on top of limitless mobility. However resource management was an issue during this content, and said bard would have to sacrifice it's own damage for the good of the team to allow TP spenders to keep spending and MP spenders to keep spending. This created a solid balance where it's power was gated and not allowed to be overbearing.
HW rolls around and as the expansion progressed, bard and machinist both saw more and more aggressive buffs. Jobs which already provided higher raid dps total contributions than casters (when combining personal damage with hypercharge/foe) as well as having access to resource regeneration tools made them provide more overall damage than both BLM and SMN, which are purely DPS classes. During Heavensward balancing patches, classes started to be almost completely TP independent. Meaning the use of Army's Paeon as a DPS balancing tool became less and less important.
So now we are in Stormblood, and I feel like we are still in Heavensward. Black Mage was released with rotation breaking issues with MP, timers too strict for the gear we had available all in all making the class feel very un-fun to play. While these issues were bandaid fixed (simply by giving more spell speed in the form of reduced cast times on f4/b4) it still didn't hold up because at the same time, ranged physical classes were being buffed as well. Machinist gets personal DPS buffs, while also providing raid utility, and even the already absurdly powerful bard gets buffs in 4.06. And now with 4.1 coming up, machinist is AGAIN being buffed while black mage gets nothing. Another important change that came with Stormblood is the free to cast MP and TP restoring abilities. Now the previously balancing factor is completely gone even further pushing ranged physical DPS ahead in viability.
In previous fights, casters were incentivised through the use of physical damage resistant mob types. The last instance of this being used was in Floor 7 of Alexander Savage, which was a very weak check capable of being dealt with by a healer (and optimally so) as the fastest kill for this turn was with both bard and machinist, in place of a caster.
So if we were to compare bringing a Black Mage and ANY ranged class right now we are trading Manashift, Apocatastasis, and Addle for significantly higher raid DPS contribution, a stronger form of MP sustain through Refresh (6kmp vs 3kmp and it is FREE), personalized forms of mitigation on par with or stronger than Addle (Troubadour and Dismantle), and the ability to do all of these while moving freely for mechanics.
3. Homogenization of the caster role.
Black Mage got the short end of the stick with the role changes in Stormblood. No way to argue it. Apocatastasis being made cross-role removed identity and value from the job. This was previously a small niche which the Black Mage could fill in coordinated teams to provide defensive utility to magic damage tank busters, or spot-apoc a low HP target party member during megaflares. Now that is shared between the other casters, which also provide more valuable contributions themselves. On top of this, the new combined manaward button took away defensive flexibility in the job itself.
Both Summoner and Red Mage have their niche in the raid scene, and out of it. Red Mage is a blind progression godsend with its simple rotation and ability to verraise at will, on top of that it is the forerunner in the oh-so-competitive world of POTD solo runs. Summoner provides very high total raid dps (which is being hugely buffed in patch 4.1) as well as a raise which is valuable for progression. Black mage provides nothing outside of its lackluster damage contribution and slow learning curve for fights due to its turret playstyle.
So after gutting the job of it's personal strengths and buffing other classes that already provided more to the team, Black Mage is left in a spot which is honestly disappointing.
We still struggle with horrible execution of rotation due to forced GCD clipping, in a world where you have such small windows to maintain buffs while executing optimal rotation.
At the end of the day you have to ask yourself why should I bring a Black Mage over any other class to any content. Casual Progression? Hardcore Progression? Speed kills?
"I hate that every time I play the job that I want, I have it in the back of my mind that I'd be contributing more by picking almost anything else" - Blackcat Ofillomen