1. a moderate to deep greenish blue.
2. one of the iron blues, a dark-blue, crystalline, water-insoluble pigment, Fe 4 [Fe(CN) 6 ] 3 , produced by reacting ferrocyanic acid or a ferrocyanide with a ferric compound: used in painting, fabric printing, and laundry bluing.
[ 1715-25; trans. of F bleu de Prusse, so called because it was discovered and first reported in Berlin, capital of Prussia ]