There is no way those should be sold as Onyx. Listings like this are one of the reasons for the dilution of the term Onyx in the hobby. At that size they should definitely be showing more black filling in. The way it's progressing looks very typical for a true-perc. While I'm not arguing the beauty of said fish (they are good looking), I think they put Onyx as a buzz-word in the posting to command a higher price.
@reppo13 - while you are correct that anemones can play an important role in clownfish coloration, true-percs/onyx seem to be opposite from other fish. Some seem to gain more intense black coloration when in a host, but onyx more frequently loose their black coloration. So this argument in this case does not hold true IMO.