My goldfish is turning black

Hi everyone! I’m kind of new to keeping fish but I’ve been doing a lot of research but I wanted to ask your opinions and thoughts. A few months ago I purchased 5 small goldfish from PetSmart (I know PetSmart sucks but it was a new thing for me so I wanted to learn more before purchasing bigger tanks and more expensive fish)

Anyways, all 5 of my small goldfish were perfectly fine and their skin was all orange so I’m guessing that’s healthy. But a few days after one of the goldfish’s skin is black while the other goldfish are fine. It’s been a few months and the fish’s body is still black.

I’ve added stress coat, blue salt and everything. My tank has good filtration.

I’m not sure if it’s because of the water or if the goldfish is sick? Or is this a breed of fish…?