Originally Posted by
Kurgath
The Black/Golden armor is because the demons are wearing it. If Demon Hunters would be just about Illidan, then there would be no sets, you would only get ripped leathery pants ...
The Demon Hunters are salvaging demons' armor, their bones, etc., they are in fact hunters, that hunt down demons and use their parts for their clothing, like actual hunters do.
Just look at Normal Demonbane Armor (Tier 20), it is based on the armor you see worn by the demon on Legion loading screen, the one fighting Varian.