The Moon Melon that has been floating around the internet lately does not appear to be real. It is likely just a picture of a watermelon that was photo shopped to be blue inside. There is a variety of melon called "Red Moon Melon", but it looks similar to a cantaloupe or muskmelon.
As for the flavor-enhancing aspect, there are a few foods which can cause sour things to taste sweet, such as Synsepalum dulcificum (miracle berries). Perhaps scientists could create a hybrid of a watermelon and a miracle berry and find a way to color it blue! Wouldn't that be something?
http://en.wikipedia.org/wiki/Synsepalum_dulcificum, http://naturalchocolate.tumblr.com/post/5033021912, http://www.clovegarden.com/ingred/melons.html