IKB 79 by Yves Klein from the Tate Collection. From their text summary:

IKB 79 was one of nearly two hundred blue monochrome paintings Yves Klein made during his short life.

Klein did not give titles to these works but after his death in 1962, his widow Rotraut Klein-Moquay numbered all the known blue monochromes IKB 1 to IKB 194, a sequence which did not reflect their chronological order.

The letters IKB stand for International Klein Blue, a distinctive ultramarine which Klein registered as a trademark colour in 1957. He considered that this colour had a quality close to pure space and he associated it with immaterial values beyond what can be seen or touched.