As far as I understand it the black markings on goldfish can dissappear as they mature. I have a few goldfish and some of them started out very black and lost it all whereas others only had a bit on and they still have it. i think its normal for the black to go and I wouldnt worry.
Alsoif the fish has been ill (ie. had finrot or something) the new tissue that re grows grows back black and will dissappear.
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