The Blackened Defias Armor was added as a cosmetic set in patch 7.1, and now every Outlaw Rogue can obtain it pretty easily. It can be transmogged by any leather-wearing class such as druid or monk once obtained, but it's currently impossible for these classes to unlock the set without making a rogue. The rogue does not need to be level 110, but it's definitely a good advantage to be high enough level to be able to solo the instance easily.
So, what you need to do is enter the instance Deadmines while in the outlaw specc. Make sure you are on normal difficulty, because it is not possible on heroic. Go to the last room in the instance where the endboss is located, and find the NPC called Ghostly Parrot. Make sure to not kill any mobs on the way over there. A few is okay, but you will have problems if you kill too many of them!
Once you locate the NPC, summon the Green Wing Macaw battle pet. This can be bought on the AH, but will also drop from the mobs at Deadmines. It's usually pretty cheap, but the price might have increased a lot now that it has a use outside of pet battles. However, if you kill pirates in the end area of the instance, you are pretty much guaranteed to get one without having to reset the instance.
When you have summoned the parrot, you will get a grapple icon on your screen. Use this to get to the NPC called Captain Bramblebeard. He will offer a quest called "Sea'in Red", which requires you to kill defias members, which are conviniently located in the very same instance you are already in. Now you pretty much need to kill and loot 100 mobs to complete the quest.
Upon getting all the bandanas from the mobs, return to Captain Bramblebeard and turn in the quest. This will reward you with an item called Ensemble: Blackened Defias Armor, and this unlocks the entire set for transmog upon using it.