This a Moonmelon, scientifically knows as asidus
This fruit grows in some parts of Japan , and it’s known for it’s weird blue color what you probably don’t know about this fruit, is that it can switch flavors after you eat it, everything sour will taste sweet, and everything salty will taste bitter and it gives water a strong orange-like taste, this fruit is Very expensive, and it coasts about ?16000 JPY (which is about 200 dollars).
Hoax or Fact:
Picture messages claim to show a rare, blue and costly variety of Watermelon called Moon Melon which is said to be found in Japan, and that it switches flavors after you eat it. As strange as it sounds and looks, the claims are hoax. The pictures are simply digital manipulations.
These messages with those photoshop pictures have been circulating since few years now. You can watch the original (not blue, but normal red) pictures in the image section below. The second picture is in fact taken from a photoshop illustration website. There are various types of melons, but not such a blue colored one, and definitely not something that can change the taste of something after you it. Nevertheless there is a hybrid type of melon called Red Moon Melon, which is a cross between a French melon and the Western Muskmelon. As shown in the last picture below, it is slightly reddish-yellow in color, and of course does not have any taste-altering properties.
However, there exist some natural fruits which have a coveted flavor-enhancing aspect. Miracle Berry (Synsepalum dulcificum) is one such fruit known to make sour foods like lemons to taste sweet. This change of taste is due to the effect of Miraculin, an active protein which is commercially used as a sugar substitute.