Higher than expected cross pollination seems to be a reasonable explanation for what you see. And there might be factors that enhance the cross pollination of Blues (more pollen, better pollen viability, increased insect visits to "blue" varieties...).
The likelihood that the non-blue varieties are "mutated", however, does not make sense to me. As a biologist, I cannot think of a mechanism that would explain a non-blue to blue mutation. You aren't suggesting this, are you?
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