Definition of soluble blue
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
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
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