a blue goldfish?

It's natural, but keep in mind that these fish are shown in blue containers with white light. My guess is that is is closer to a slate gray color, and looks blue b/c of the show container.
 
Chris and Somer are right. They look more slate gray usually but they're still classified as blue anyway. I actually adore those orandas particularly lavenders.
 
There is a blue goldfish out there, the blue egg phoenix. While a beautiful goldfish it is impossible to get a hold of because there are limited numbers in North America... and most of the breeders wont give out fish to just anyone
 
Natural, but not entirely likely to keep its color. Blue in goldfish follows the same rules as black, chocolate, and lavender. Namely, the larger the fish is, and the stronger the intensity of color on the belly, the likelier it is to persist for a long time. The lighter colored the bottom half of the fish is, the further the demelanization has progressed. Many of these black based color fish will end up predominantly white and orange unfortunately, though others do keep their color throughout their life. Also, I would caution against buying any goldfish based solely on a picture. You really need to be able to see the fish swimming, and in this case see its appearance from the side since I assume you'll be keeping it in a tank. Many goldfish photograph beautifully from the top down, but look crappy from the side, or have serious swimming issues you can't pick up on in a photo.
 
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