noun
1. often capitalized S&B : any of several water-soluble acid dyes made by sulfonating the Spirit Blues and used in the form of salts chiefly in writing inks, in laundry blues, and as biological stains — see dye table I (under Acid Blue 22 )
2. : a pigment dispersible in water that is made by treating an iron blue with sodium ferrocyanide or oxalic acid and is used chiefly in permanent writing inks and laundry blues