Provided are indigo darkener, containing: in parts by weight, 0.1-99 of indigo dye, 0-99 of alkaline dyeing assistant, 0.01-99 of acidic dyeing assistant, and 0-99 of oxidizing agent, and hair dyeing process using the indigo darkener. The multi-layer scales on the surface of hair are opened at large angles under alkaline condition and/or high-temperature condition, so that large indigo dyeing particles are easily absorbed in the scales; black indigo oxocarbon or carbon black is generated under the action of alkaline dyeing assistant and oxidizing agent at high temperature by the property that the indigo dye is easy to oxidize; the acidic dye assistant is added as the temperature decreases, and the scales having opening angles on the surface of hair are closed under acidic condition, so that the indigo carbide is firmly clamped and fixed in the scales of hair; finally, the blue indigo dyes white hair into black.