I've seen sets which the cent had black corrosion, what caused this ?
I don't know for sure but probably plating problems and it's the zinc underneath turning black.
If you mean sets in the original US Mint proof set lens then I believe that the plastic on these sets could sometimes degrade and leave bad looking black spots or debris on some coins.
In 1984 the mint changed to a paperboard insert to hold the coins instead of the previous plastic one. The Sulphur in the board attacked the copper plating.
Several things, poor quality control and materials from the mint, poor storage conditions, or a combination of these and other factors.