noun
1. : a soluble blue dye that is the sodium salt of indigodisulfonic acid and is used chiefly as a biological stain and food color but is no longer used to any extent as a textile dye — called also Indigotine IA
2. : a strong greenish blue that is bluer and duller than grotto, greener and duller than cobalt blue, and greener and darker than average cerulean blue (sense 1a) — called also chemic blue, duck blue