File:Bluemoon.jpg

Linked as a reference on Moon.

Summary
This information was presented as coming from a Blizzard Game Master and it should be noted that it has not been made clear if Blizzard representatives hold the authority to create lore, though they may clarify it, so this information may or may not be canon. It is also possible that the screenshot is not authentic and legit at all, so this information should probably be taken with a grain of salt until confirmation.

Regarding an official response about the disappearance of the Blue Child, the GM said it has to do with the Tauren myth of Mu'sha - the story of the moon and how the blue moon "Blue Child" is the left eye of the Earthmother, which represents imagination. Because of the coming of the Burning Legion in The Burning Crusade expansion, she sent the Blue Child away to learn about the universe and it has been gone ever since, much like its mysterious disappearance from the sky after Vanilla. The GM also stated that Blue Child may not be returning for a long, long time.