Concerning the black cover, there's a machine in the dungeon usually on the same level as the pit that gave you the black cover that will only give an option to someone that is covered in the black goo. it says it will remove it. As to the Divine set it's basically luck as to whether or not you get the real one or as Secretsakura stated, the ones that come from dungeon completion rewards are always what they say they are so just keep an eye out for it as a reward.