Just some very basic information here:

Black spot is the name given to either of two goldfish illnesses:

1) In fact the first one isn't a goldfish illness at all, it is the sign of healing from injury caused by ammonia, rough handling, low pH, skin flukes or goldfish ick, and can appear on the skin or fins.

It is important to note that the black spots will not appear until the problem is removed and the skin begins to heal. This type of black spot usually disappears within 7 days.

2) The second type of black spot is a parasitic goldfish illness. Pond goldfish and those living in aquariums with snails are more at risk. Small black or brown spots about 2mm in diameter appear anywhere on the goldfish, including the mouth. They contain the larvae of parasitic flukes. Consult a pet shop or vet on the best anti-parasitic medicine for this goldfish illness if it becomes a problem.

Even after the tank and fish are free from the parasite, this type of black spot can remain on the goldfish as a scar.

SOURCE: http://www.happy-goldfish.com/black_spot.php