It would seem to me that crossing any varieties would result in changes throughout a plant's structure and fruit with the introduction of various different genes resulting from the cross, regardless of whether the cross involved a blue variety. For the past 3 years I have been growing dozens and dozens of varieties together, including blue ones, and I have seen no blue seepage into others. I've seen very little crossing at all from any varieties. Perhaps I don't have the number of insect pollinators causing crosses as others do, although I have bumblebees at work constantly in my tomato patches. I will pay more attention in the future.
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