How do you say Blue Skies in Latin?

I'm going to recommend caeli caerula, but there's a lot to consider.

The classical Latin Word for "sky," caelum, is neuter, but Old Latin it was masculine caelus, and it retains masculine gender when it appears in the plural (rarely, and poetically, in classical Latin but commonly in Church Latin, where it means "heavens").

If you're writing classical Latin prose, what you get is a blue sky, caelum caeruleum (or caelum caerulum; the spelling of the adjective varies).

If you're writing classical Latin poetry or Church Latin, you might prefer blue skies/heavens, caeli caerul[e]i.

Or you may perfer to split the difference by going with the recommended phrase above, which is from De rerum natura ("On the Nature of Things") by the classical poet/philosopher Titus Lucretius Carus. Caeli caerula is literally "the blues of the sky."