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Hi menearb,
It's important to know how large the goldfish is. If his total length (including his tail) is over 2 inches, then the ten gallon aquarium is too small for him. He really should be in a 30gal tank.

If he's too large for his 10gal tank, he is producing more ammonia than the beneficial bacterias can handle. The beneficial bacterial colonies, if they are established in a fully cycle tank, will change toxic ammonia into nitrites, nitrite into nitrates.

The discoloration may be cause by ammonia burns. The dark areas could be an indication of healing or your goldfish may be experiencing a color change.

For more information about cycling your aquarium read.... The Nitrogen Cycle

Knowing what the water parameter test readings for ammonia, nitrite and nitrate is very important.
The test kits will help you determine if the tank is fully cycled.
When the tank is established, testing for nitrates will help you decide how much of a water change will reduce the nitrate reading to under 20ppm, (safe range = zero to 20ppm)

Are you cleaning everything within the tank, including the filters?
Cleaning everything will destroy the beneficial bacterial colonies that are living on the surfaces of the filter, filter media, gravel, glass, gravel, and decor, etc. If you clean too thoroughly, the bacterial cycle will begin to cycle again, after every cleaning. The tank will never be fully cycled / established.

Siphoning the waste out of the gravel with a python siphon/gravel cleaner will prevent toxic pockets of gases from forming in the gravel. You can also prevent this by removing the gravel.

Rinse the filter media with tank water. Chlorinated tap water will destroy the bacteria living on the media.

Reduce the amount of flakes. Feed only a very small amount (2-3 flakes) twice daily.
Sinking pre-soaked pellets will prevent him from swallowing air from the surface.

If your goldfish still wants to eat, I would try feeding him a frozen>thawed or fresh>blanched deshelled small pea.

You can add 1 teaspoon of pre-dissolved non-iodized salt per gal. 10 teaspoons for a 10 gal. tank. This will help prevent nitrite poisoning.

Finally, we don't know how old the goldfish is. If he has been healthy up to now, he may be at the end of his life.