(from Latin: caeruleus, “dark blue”). (Also spelled caerulean). In the late 18th century Swiss chemist Albrecht Höpfner created the synthetic pigment cerulean blue from cobalt stannate. However, it did not become commercially available until the 1860s. Cerulean pigment produces a fairly true blue, ranging from light azure to a more intense sky blue. In oil paint, it was particularly valuable to landscape painters (for skies), and it was popular with Impressionist and Post-Impressionist painters. Web resources here and here.

Édouard Manet. Corner of a Café-Concert. Oil on canvas. Probably 1878–80. National Gallery, London.