Black marks have appeared on goldfish. One has died, others have the marks!!

Hi
This morning one of our goldfish was dead. Other than 2 black marks on
his head he seemed quite normal.
Our pond is a garden pond about 6 x 4 x 4. It contains about 40
goldfish and apart from cleaning and feeding the pond is left very
much alone. The pond has been going for about 10 years with no serious
problems. The thing that worries me now is that 2 other fish have
black marks although they look quite healthy. I know I have seen
discoloration before on the fish so I'm not sure if the black marks
are related to the death of the fish.
Any help much appreciated.
Thanks.