Blackened Dragonscale can be skinned from dragonkins in Cataclysm zones only. Blackened Dragonscale can be skinned from from a wide range areas, but the best way to farm Blackened Dragonscale is get the quest “Blackout” and never hand in the quest. With this quest you will receive “Wyrmhunter Hooks” that can be used to hook the drakes that is flying above down to the ground where you can slay with ease.
If that is not an option because you have completed the quest earlier, then head over to Deepholm and farm the Stonescale Drakes located south east of the map, right below Storm’s Fury Wreckage. The drakes are level 81-82, not at a really high level so easy to slay and skin.
See below for maps to farm Blackened Dragonscale in WoW.