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“eats through” the enameled surface of lids and the hydrogen sulfide is deposited on the eaten through places causing a dark spot.
It is not harmful unless, of course, the add has eaten all the way through the lid. That would mean the jar is no longer sealed and there are likely to be more serious problems.
Some brands of lids seem more resistant to citric add than others.
VPI supports the USDA recommendation for canning tomatoes and tomato juice by the hot-pock method is a boiling water bath canner. This method is safe although the product may be overcooked.
An alternative would be to recommend the processing times for pressure canning of tomatoes from the study by Zimmerman et al.
Pressure Pints Quarts 5 pounds 20 35 10 15 25 IK 5 15
75.5%