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...
in greek mythology the sky is blue because the Greeks beileve that afrodity the greek goddess of beutty spilt her prettios blue perfume on the clouds and it slid right off and stained the sky