(a?) clear blue sky

[joham]

For most of the journey there was clear blue sky.

This sentence is taken from Cambridge Advanced Grammar In Use. Could we add 'a' before 'clear blue sky'?

Thanks a lot.

[BobK]

The meaning would be different. If you say 'a clear blue sky', you are referring either to the sky at one particular moment, or - idiomatically - to an unexpected event.

There was clear blue sky throughout the journey.
but
There was a clear blue sky when we set out, but the clouds got thicker and thicker as we neared the camp-site, and we had to put the tent up in pouring rain.

And, idiomatically:
We were getting on fine, when out of a clear blue sky she suddenly started crying.

b