cerulean (adj.) 1660s, with -an + Latin caeruleus “blue, dark blue, blue-green,” perhaps dissimilated from caelulum, diminutive of caelum “heaven, sky,” of uncertain origin (see celestial). The Latin word was applied by Roman authors to the sky, the Mediterranean, and occasionally to leaves or fields. As a noun, from 1756.
What a wonderful, colourful word that can be applied equally to sea and sky, to leaves and fields. I associate it with Titian blue, the blue that frames the hapless, helpless cries of Ariadne as Theseus sails away and the moment in which she glimpses the leaping figure of Bacchus, new-sprung from his chariot to rescue her. Look at the blue of the water, of Ariadne’s robe and then the blue of the sky above – in which she already exists, transformed, as a crowning constellation of stars.