There’s often a chasm between conceptual art and the rest of the world. But International Klein Blue (IKB), a colour devised in 1957 by French artist Yves Klein, is having a high-tide moment in couture and décor. IKB featured on Oprah, and since then fashion has pulsated with this fathomless indigo hue, with designers from Giorgio Armani and Jil Sander to Prada, Kenzo and Stella McCartney falling for it. You can even buy IKB sunglasses from Etnia Barcelona. Klein exhibited monochrome paintings, later patenting his formula and famously using it in his Anthropometries – filmed performances in which naked women rolled in IKB paint. Valeria McCulloch, also known as ValBlu, has worn IKB every day for the past 15 years and regards Klein as “a genius – the first artist to use a whole human being as a brush. Lots of designers have used IKB for inspiration: look at Yves St Laurent’s Jardin Majorelle in Morocco.” Part of IKB’s allure, she believes, is that deep blue has always been expensive. Ultramarine, made by grinding lapiz lazuli mined in Afghanistan, was prized in the Renaissance. And in the Christian tradition, dark blue has often denoted divinity.