On my Dutch forum someone had a very red concinnus that died of melanomas. And later a anerythristic concinnus also had a melanoma.
Both animals were 6 years old, but not from the same litter. Do not know if they are related, but will ask him.
He went to a vet who performed an autopsy and found that the cancer had already spread considerably. Melanomas occur often in black snakes was his conclusion.
According to the vet (after he consulted some other specialists) this could be prevented by using UV light.

Personally I have had many black snakes in the last 40 years and I have never had one (or took care of one) that had melanomas.

What is your experience in this?