While I agree its normal, its actually kind of rare. I believe you have a dirty tail clown or dusty tail clown. I can't tell if its a true percula or a occy... but that particular trait has turned up more often in occy. But not to say that it hasn't shown up in percs.

P.S.
CoralAddiction, I HAVE seen the bluish-grey sheen on the rear ends of some percs, but you could have shown a better example of the rare occurrence. The percs from your screenshot don't exactly meet the criteria of the clowns we're discussing ATM.
If you HAVE seen it may be you can post a better sample
Picture yourself instead?