buddleia in a dappled sunset

If only in one patch, then more likely an anomaly in your vision.

I don't think so. Very blue skies do occur in nature; and this was probably the bluest I'd ever seen or ever will see.

With the possible exception of the Bryce Canyon in Utah, in November 2015:

Blue-sky photos do sometimes come out that colour,
but whether it's actually that colour is a matter of perception.

Colour temperature can be measured objectively (independently of how we perceive it). For studio work, professional photographers use a colour temperature meter (similar to a hand-held exposure meter).

Measured with such a device, the sky in my shot, above, and the actual sky would have been close.