Black mages can get exp invites at 75, just most/all of them do manaburns to get exp. Black mages only getting exp through manaburn is as often as not due to the choice of the black mage more than the choice of the party. I've seen more black mages refuse non-manaburn parties than I have black mages who have accepted a 75 party invite. In several exp camps you can't do a cannon party, so a black mage is still useful, they can exp there. A lot of blm's just don't choose to get normal parties, so saying manaburn is the only option is just false.
I'm sure that it's not going to be a one blm cut-off point and then they're just useless; seriously stop making it out to be the end of the world. For big alliance fights, it will likely be 3 or so black mages (i.e. 1 per party) for completely unhindered damage. It will also likely be a diminishing returns based on the number of blm's you have in the alliance at a time. 2 in a party, they might be at 90% each, 3 might be 75%, who knows but it will be a system similar to that. If SE makes it so 2 blm in a single party reduces both to 50% strength, then I will agree with you that they're severely hurt.
The new spells are also included to deal with them being weaker in large groups. If the new spell is 20% stronger, but they are at 70% strength because there's 2-3 in the party, that's still 90% as effective as solo before the patch. That's a decent trade-off, they get weaker as a result of the patch but there's something so they're not totally useless.
As for the MPK patch, that wasn't targeted directly to BST's, so it's irrelevant. The patch was designed to hinder MPK attempts, and they most likely discovered the impacts on BST during the testing of the update. That would be why they implemented the changes in BST, because they saw the impacts that it would have on a job based on a descision they were committed to. As for it being the "lazy man's way" it was actually asked for by players. Stupid players, sure, but it's not like they decided to pick the one solution that woudl hinder a job the most.
Warrior and Monk aren't overpowered in endgame, and exp wasn't what SE meant by overpowered; so they're not likely to be nerfed.