Prussian blue

WordReference Random House Unabridged Dictionary of American English © 2025

Prus′sian blue′, 

1. Fine Art a moderate to deep greenish blue.
2. Chemistry one of the iron blues, a dark-blue, crystalline, water-insoluble pigment, Fe₄[Fe(CN)₆]₃, produced by reacting ferrocyanic acid or a ferrocyanide with a ferric compound: used in painting, fabric printing, and laundry bluing.
• translation of French bleu de Prusse, so called because it was discovered and first reported in Berlin, capital of Prussia 1715–25

Collins Concise English Dictionary © HarperCollins Publishers::

Prussian blue noun

1. any of a number of blue pigments containing ferrocyanide or ferricyanide complexes
2. the blue or deep greenish-blue colour of this pigment
3. (as adjective): a Prussian-blue carpet