turnbull's blue

from The Century Dictionary.

• noun A species of Prussian blue which is thrown down when potassium ferricyanide (red prussiate of potash) is added to a solution of a ferrous salt. When dry it has a beautiful blue color with a reddish luster.

from the GNU version of the Collaborative International Dictionary of English.

• (Chem.) The double cyanide of ferrous and ferric iron, a dark blue amorphous substance having a coppery luster, used in dyeing, calico printing, etc. Cf. Prussian blue, under Prussian.