the blue / green line by the dorsal fin comes later . My black was 4" before he started to show it . the fish in the OP to me look like Scopas..
^
how do you differentiate it from full scopas and scopas/black hybrid..?
A scopas/black tang hybrid will usually have a maze of small blueish lines across it's body. True black tangs are solid black or dark gray at that size. A better picture of your fish is needed before a distinction is made. If you are having a hard time catching it, just net it at night when it is sleeping.
I would have to agree with you and for the price should be no doubt imoChris, yours looks different than the ones the OP put up and has the look of a true black tang. I dont think RT would have done that with any malice at all considering the amount of stock they probably move. They (RT) were probably sent a batch of "black tangs" and sold them as such. Its unfortunate that some may not be full 100% z. rostratums.