I learned the hard way goldfish sometimes get black color to their fins and such as a result of ammonia burns
The color actually shows up when the water quality improves, so if you aren't monitoring your water quality you won't know the ammonia was at a dangerous level until the damage has been done. I think there may be some fish fungi that cause black splotches too, but I'm not certain.
I have a goldfish that I've managed to keep alive for 4 years. I originally bought her as a 9-cent feeder fish for a lizard I had at the time, but she & the others had ich and after I treated them, didn't know if they would be safe to feed. So I kept them. As time went by, the others died, until there were two left who each grew to be about 3-4" long. The other fish died of what I think was a bowel obstruction, she was lethargic and bloated for days, and "Fishie" would actually push the sick one up to the top for food when I fed them!
And now, there's just Fishie. I kept her in a filtered 10-gallon tank for 3 1/2 years, and she got so big that I finally splurged on a 45-gallon tank. She's about 8" long now and looking good! I took some pictures of her when I moved her into her new home
If your son has them in a fish bowl, I would recommend getting a filtered aquarium- you can buy a ten-gallon set up for pretty cheap at places like WalMart & PetCo. I'd also invest in some Aqua Safe or something similar to help maintain the water quality, and an ammonia test kit.
If a bowl is the only thing you can offer the fish right now, frequent water changes will be necessary, and you could try the "Bowl Buddies" fizzy tablets for maintaining water quality.
Good luck to you & your son with the fish! I get a lot of joy watching my Fishie swim around- it's very soothing after a stressful night at work!