Blue hair dye black hair is a hair coloring technique involving the application of black hair dye to previously blue-dyed hair. For example, a person with naturally brown hair may dye it blue, then decide later to dye it black.
Dyeing blue hair black offers several advantages: correcting uneven blue hair color, concealing blue hair growth at the roots, and achieving a darker, more professional appearance. Historically, black hair has been associated with strength, power, and mystery.