I checked out www.askjeeves.com and one of the sites had goldfish diseases listed. Here is what it said about black spots. You might want to do some internet research, but I thought this might give you a little information.

BLACK SPOTS
The most common cause of black spotting on fish is transient ammonia spikes (or other toxins) that cause burning. Check water parameters first, do big water changes. Black is a sign of healing. Spots should fade in about 2 weeks.
If these are new fish, if there are snails in the pond then it is possible the fish have "black spot disease".