Why does your canned tomatoes turn dark on top?

it means the tomatos are gone of

Not necessarily true. According to the Penn State Extension office:Seed Discoloration - Canned TomatoesA consumer brought in a jar of home canned tomatoes to the office (canned last year). Some, but not all of the seeds have turned black. Is there any danger in using these tomatoes?

ANSWER - If the seeds began to discolor during storage, it is probably not a sign of spoilage as long as the product was processed correctly and a vacuum, indicating a tight seal, has been maintained. It may be a reaction of harmless polyphenol compounds in the seeds with iron or other minerals in the water.

http://extension.psu.edu/food-safety/food-preservation/faq/seed-discoloration-canned-tomatoes

Accessed January 25, 2013.