Hidden Artifact Appearance Elemental shaman

[jale]

Guys,i have a problem,i dont have the initial color variation for the hidden artifact appearance but i managed to unlock the others by completing challenges for them(how could i unlock other 3 color variations if i havent unlocked the initial one?). I farmed like crazy those npc in Dalaran Sewers but i didnt get anything. Is that drop somehow bugged for me or what? I killed a lot of those npc's and i got nothing,i have 5k Sightless eyes and those Spellsludge drops from the rares,and no item. If anyone knows what should i do,tell me. Thank you!