How big is it? Percs can take a very long time to fill in their thrird bar. From the pic it seems to have the body and head shape of a juvi +/- 3/4". Totally a guess, but if it's small like that, then the 3rd bar will likely grow in within the next few months. If it's older like 1.25" then there's a good chance it won't fill in. Beautiful fish though either way!!
As far as what it is...it's a percula. Onxy is a name coined by a breeder and is now often used to describe just about any perc with lots of black(melanistic). I keep hearing black percula, but as far as I know, that term doesn't refer to any specific genetic lineage. At your percs age which I'm still assuming is very young, maybe 6 months, It has VERY nice coloration, and is definately a beautiful specimen. Also if it is a juvi, that black will likely continue to fill in and become even more pronounced.