parrot fish

I wanted to ask something about our parrot fish. We have 5 of them. We've had them about a year now and they're all growing and getting bigger and seem very healthy. Each one of them had some black on it when we first got them, but as they grew and got older the black went away and I figured it was the chlorine doing it. But, ever so often one of them will get some black coloring come up on their tail/face. It's strange...and then a week or so later it'll fade away...and then it will usually happen again within a month or so. Is that normal?