I posted this earlier today, but I'll post again here:
The term Onyx Percula was originated by Bill Addison at C-Quest. Only those fish with a lineage of the original C-Quest clowns can be called Onyx Perculas.
However, these days the term "onyx" commonly refers to the melanistic form of perculas where the body contains a lot of black, specifically filling in the body between the first and second stripe, as well as the second and third stripe. Some people also believe that in order to qualify as "onyx" the fish must also have a black dorsal, but this is up for debate.
There are some descendants of the original Onyx Clowns that are still called Onyx -- Rod's Onyx and Booyahs come to mind.
Furthermore, there have been wild onyx clowns, as well as other onyx clowns bred from these wild varieties that are called onyx clowns (notice I don't capitalize it).
Lately I've also seen the distinction of "super onyx" and "premium onyx" where the tail is completely black. I personally like the balance of orange and black on the original Onyx, where the head and tail still have a decent amount of orange. In other words, if this were a competition and were a judge, I personally would downgrade a clown that had a black tail since I believe an Onyx Percula should have an orange tail. Too much black and they start looking like Darwins. But again, that's just my personal opinion. Since we don't have an actual set of "laws" to follow, as we do with koi for example, this will continually be up for debate -- and the range of prices will reflect personal opinion on worth.