http://disneyinfinity.wikia.com/wiki/Pitch_Black_Sky ^Includes description of unlock method, which I'll repost below^ To unlock the sky, you need to go to the Introduction to the Toy Box Toy Box. From there, you must simply wait near the main area until a Yeti Costume spawns. This can take anywhere from a few minutes to a hour or two. Once the Yeti spawns, simply pick it up and it will explode into Sparks. There will not be a notification, but the sky will become an available option for the Sky Changer.