That is very normal...they are gaining their black. I've noticed that they don't change to the full onyx black coloration like normal clowns. Yes, it starts from the bars, but they get really blotchy when they start their coloration. Do you see the part of the body right above the pectoral fins gaining some black splotches? This is where mine started getting most of its color. As long as it is nore black than red, should not be any problem with the fish, just growing up. Not a hosting problem, should not be disease.