OK, sorry to revive an old thread but thought I would add me .02 cents here...
I picked up a little baby .25" Onyx clown (that was being hosted by a RBTA) at the LFS... We picked him because he had a bunch of black on him, a lot more so than his siblings. We've had him since May now, and he has zero black on him... He's lost everything that he had...
What is going to be interesting is that he has now taken up residence in a RBTA in my tank, so he doesn't spend as much time in the GBTA.... We'll see what happens in the coming months, if any of that color comes back or not. He's about .5" now.... I'll post updates as things happen.