This exploit won't help you beat about 75% of the game on Dark, but it will ease your pain when you try to keep the Dark Outfits and fight the dragon (or the Operator) after you already have the Mad Man achievement (which requires you to beat the whole game on Dark ... so this exploit really is more for people who make Witcher 2 videos).
When you change your difficulty from Dark to Easy, Normal, or Hard, you lose all the parts (and diagrams) for the Dark Outfits if they are in Geralt's inventory. However, the game does nothing to the items in your storage box, so ... feasibly, you can play the game on Dark up to the point in Chapter 3 where you purchase all the Kinslayer diagrams from Bras of Ban Ard. De-equip and store all of your Dark Outfit parts (armors, weapons, and diagrams), save the game (just in case), change the difficulty, and take your stuff out of the box and continue playing.
Final note - this exploit is valid for the launch version of The Witcher 2 Assassin of Kings Enhanced Edition on the Xbox 360; the PC version, and future Xbox 360 versions, may be patched to rectify this oversight.
Keywords: The Witcher 2: Assassins of Kings (Enhanced Edition), The Witcher 2: Assassins of Kings, The Witcher 2: Assassins of Kings (Dark Edition), The Witcher 2: Assassins of Kings (Collector's Edition)
Tags: pc, the-witcher-2-assassins-of-kings-collectors-edition, witcher-2-dark-outfit-inventory-exploit, games, the-witcher-2-assassins-of-kings-enhanced-edition, xbox-360, the-witcher-2-assassins-of-kings-dark-edition, the-witcher-2-assassins-of-kings