Answer: Although I don't think anyone knows for a fact, I suspect the lack of blue flowers may be related to what insects find attractive, in terms of plants needing pollenators.
The blue vs. purple label is always an interesting question. It is true that some flowers will change color according to time of day, soil type, temperature, moisture, and so on. The bluer early morning light and bluer northern light can also contribute to a perception of blue over purple. It is also true that wishful thinking and "eye of the beholder" seem to have something to do with it.
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