I was looking at the latest snapshot that was released in a single player game. and I was in Creative so I could easily look at all the new stuff. I was looking at the Extreme Hills, I loved the new generation for it. When I flew under an overhang though, the edges of the sky started turning black, and I thought a Wither spawned somewhere for some reason. There was no Wither though, it was just the overhang. I flew out from under the overhang, and the sky returned to normal. I experimented with this, and learned the more blocks covering the sky, the darker it gets. I took some pictures to show.

To remake this, just put a block above your head, it doesn't matter how many. Then look off in the distance. The edges of the sky will look darker than normal.