Black markings are a definite indication of something called ammonia burn which is basically a reaction to ammonia in the water.
As you are most probably aware, ammonia is harmful and often lethal to fish and should never be present in an established tank.
The best way to eliminate ammonia from the water is to do a series of daily 15% partial water changes in combination with a drastic reduction in the amount of food being fed to the fish.
You mentioned that the fish's body is round. If this is not the fish's normal shape then it could be an indication of swelling. Once ammonia and it's follower nitrites (if now present or if they become present after ammonia is eliminated) have tested at -0-
and if the fish's body has not returned to normal then add some epsom salt to the water at a ratio of 1 teapsoon per every 5 gallons of water as the salt will help withdraw any accumulated fluid the fish might be retaining. The use of an antibacterial medicine such as Maracyn-Two can also be used in conjunction with the epsom salt.
Best wishes and let me know if you have any questions.
.
Is the black eye from the ammonia burn as well?
Do pet stores sell Maracyn-Two?
How does drastically reduced feeding help with ammonia in the tank?
There is a second goldfish in the tank, would it cause any issues if it came in to contact with the epsom salts or medication?
As I can't see the black eye I'm unable to say for sure if the problem is related to ammonia burn or a bacterial problem hence the suggestion to use Maracyn-Two.
Yes most major pet stores in the USA sell Maracyn-Two. If you're located outside of the USA let me know and I'll recommend a medicine easier to find.
Cutting down on the amount being fed will reduce the amount of food the fish eats and as a result reduce the amount of fish waste and uneaten food which the fish does not eat which will decay in the water. Both fish waste and decaying matter are fuel for ammonia growth.
It might hurt the other fish to come in contact with the salt so it would be advisable to seperate the affected fish and treat with the salt but please keep in mind that ammonia and nitrites must be eliminated before any type of treatment begins.
YOUR TROPICAL FISH RESOURCE
OVER 20 YEARS EXPERIENCE W/TROPICAL FISH---BREEDING, MAINTAINING, CURING