Hi, Colleen,

A small amount of black residue on the inside surface of lids of jarred salmon, is perfectly fine.  This is very common and is probably iron sulfide that is produced when salmon is heated to high temperatures and there is a reaction with the metal of the lid.  Even though the flat lids are coated with a lining that should keep the metal lid from reacting with the fish proteins, there may be very tiny breaches in the coating that allow this to happen.  

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