Say hello to my Black Priest outfit. Straight from the depths of god knows where as I just really, really wanted to wear those pants.
The staff I believe is an artifact appearance version but there are plenty of dark staves and staffs that can be used in place of it if it’s one that you cant get hold of.

Head: Cowl of Absolution – Yes its navy blue but it is dark enough to get away with. Token drop from Archimond in Battle for Mount Hyjal 25 man (Legacy raids now just seem to automatically set to 10 man if you are on your own, but still seem to drop anything for both sizes, which means that the drop rate for the token has now gone down….).
Shoulders: Mantle of Absolution – Token drop from Black Temple, Demon Ho with the million arms. She alto drops the companion pet too.
Back: Hidden, need to show off those cheeks!
Chest: Master’s Vest – Try Molten Core for this one, although there are other black coloured chest items around if you can’t get the bugger to drop.
Waist: Hidden, the buckle on the vest was enough, it all looked a bit bulky with anything else.
Legs: Metro’s Slimming Legs – Quick two quest series end reward from Blades Edge Mountains so you don’t have to wait for a shitty drop rate for these bad boys!
Feet: Coldwraith Boots – Wrath of the Lich King dungeons for these. I used them so they didn’t detract from the look of the legs, although, any other black boots will work just fine and you might have some of those already.
Gloves: Heroes’ Gloves of Faith – This is from Vault of Archavon in Wintergrasp, so its a bit of a shitter to get as your faction has to control the area… But! you could always opt for the ever classic Black Mageweave Gloves and some appropriate bracers to top them off.
Staff: Atrifact weapon appearance is the one where you have to get a drop of one half of the staff from the Halls of Valor dungeon and then buy the other half after hitting exalted with the Valarjar. This particular colourway requires the achievement of completing 30 dungeons once you unlock a hidden artifact appearance. But, if you cba doing all of that then there are some awesome black staff options that are around. My other one before getting this colourway was Formidable Spire which you can get (and any of its name variants) from missions in your Garrison. Or… This, This or This.

Don’t forget there are more transmog/slutmog that can be found here.