As funny is this sounds, from my experience and looking at other WoD drop tables, this is the order of rarity:
1. Gloomshroud Armour (Rarest SFK drop).
2. Shadowfang.
3. Assasins Blade.
4. Guillotine Axe.
5. Night Reaver.
Now they could all have the same low % drop rate, its all RNG so you never know, but that's from my experience/knowledge.
Its funny Iv'e seen Shadowfang, Assasin's blade and Night Reaver drop all in one day (in under 15 runs), yet I've been farming Foremans Gloves for 3 days and no success, I made Foreman Thistlenettle drop around 15 of his blue drops and none of them were Foremans Gloves, Does it even drop anymore?
And when I do Shadowfang Keep I que for it in the dungeon finder, group queing with randoms increases the drop rate, Blizzard feels bad since your qued with 4 other randoms that might take your loot, so they feel the need to Amp up the RNG drop rate a little bit. to make it easier. Only bad thing with this is you get noobs needing shadowfangs off of you, which just happened to me.
Goodluck farming X.