I'll try and get some better pictures when I get home.It looks like bad fin damage to me. A double tail would split all the way to the caudal peduncle. Need a full flare photo to determine if he's a half moon or delta tail.
He's a beautiful fish. I couldn't imagine him being culled for a defect like that, if it even is one.I'm not sure, but looking at these pics he may be a double tail that didn't develop all the way and instead of being culled he made it to the pet shop.
Try putting a mirror up to the tank to get him to flare fully. He's a very pretty blue.
It would be easier to tell if I had more input. So far it's just two people with two different answers.