Black marks on Fantail

[Reggie]

I have just noticed that my one of my Fantail goldfish has developed a couple of black marks on her dorsal fin. She has also had one come up on her tummy. Is this normal?? I have spoken to the shop where I got the fish and he said it was quite normal as it is just pigment. She is swimming and eating ok. I am just after a bit of reassurance. I would hate to think it was something more serious. Thanks