Michigan State University Extension
Preserving Food Safely - 01600681
Black spots on the underside of the lid does not mean the food is spoiled. Naturally occurring compounds in foods including tannins, hydrogen sulfide or nitrogenous compounds can react with metal lids causing black spots. These reactive compounds are associated with proteins in the food. When subjected to heat of processing, proteins denuture and these compounds are released. Once freed from the protein, they are able to combine with the metal lid causing brown or black deposits.
If the jar of food is no longer sealed (vacuum lost), black spots on the underside of the lid mean that the food is spoiled. In this case, the contents of the jar should be discarded.