In the End, endermen are spawning regardless of light level, and they are spawning on top of stained glass blocks. I've tested this by completely covering everything in the End (the main island where the dragon spawns) with water - after the dragon is dead. Endermen will still spawn on stained glass blocks that I place down, regardless of how well lit the area is. They are also spawning on top of the bedrock (fountain and blocks on top of the obsidian pillars) if I don't cover their surface with bottom-placed slabs or water.