You can finally get Grotto Netherwing Drake: Black Scales in Patch 10.2.6! Like many other mount rewards added in the patch, this is from a meta-achievement.
Grotto Netherwing Drake: Black Scales was originally datamined back in Patch 10.2, as part of the Maruuk Centaur reputation. However, that never made it to live.
This will unlock a new skin color for your Grotto Netherwing Drake! You can preview how it looks in our Dressing Room.
The reward is now unlocked from completing a new Dragonriding meta-achievement, Now THIS is Dragon Racing!, requiring you to collect all the glyphs and complete all the races.