I personally don't see anything shady about that photo. Chnces are good that wether it was taken with or without flash, that if it had any irridescence at all, it would have showed up.
Most blacks you'll see on aquabid have a bit of a blue sheen to their body becuase of the fact that you have to breed a black melano to a steel female that carries the melano genotype. Sometimes you can find a betta, like that one that doesn't have that irridescence, or has very little, which makes it a fine specimine for the melano type.