My fish had black stuff on it's back and now it's gone?

I noticed about a week ago that my goldfish was developing a black line across her back, along her spine. I was concerned because I thought it might be a fungus. I asked the aquarium attendant at the pet store and he said that she might just be maturing and the black stuff was just colouring. It is now a week later and all this black stuff has gone and she is back to normal. Should I be worried there is something wrong with her? She isn't acting any differently.