First off, the Heart of Sabik, which was what allowed the Ultima Weapon to cast the titular Ultima magick, is not an artifact of Mhach (the civilization which birthed Black Mages). It predates even the Allagan Empire, for that matter. As such, in FF14's universe, the Ultima magick bears no relation to the practice of Mhachi Black Magic, which is the discipline player Black Mages follow. In addition, the Ultima magick itself proved devastating enough to Hydaelyn herself that she couldn't shield us from a second cast of it. Wielding such raw power at the frequency with which Black Mages tend to do such things at would likely mean the end of the world over the course of two minutes. All this is, of course, a very roundabout way of saying that BLM is never getting Ultima, because it would contradict pretty much everything previously written about FF14's universe. Also, Shadowbringers has nothing to do with time travel, no clue where you got that. We're traveling to a different Shard, that's all.