I'm drawing a blank here, did Shadowlands or its pre-patch ever have a known black dragonflight story that was scrapped in the development process?
Was looking back on the Zandalari/Kul'tiran hints in Azuna from Legion launch, turns out the SL pre-patch added some seemingly unusued dragonkin typically associated with the black dragonflight:
Obsidian Deprivator & Molten Scaleburner, using the Maloriak/'dragonman' models.
Two 'Fire Dragons' which use the Atramedes and Nightbane models.
Smoldering Monstrosity using the weird dragon-zombie-abomination model.
There were also two enthralled stormdrake mobs that were added, they could be entirely unrelated though.
Not that this would tell us much since I assume they were never actually added to the game itself - no location data or anything in Wowhead/WoWDB. Just interesting since we know that they can get cute with in-game hints as early as the launch of the previous expansion.