The present invention provides an indigo darkener, containing: 0.1-99 parts by weight of an indigo dye, 0-99 parts by weight of an alkaline dyeing assistant, 0.01-99 parts by weight of an acidic dyeing assistant, and 0-99 parts by weight of an oxidizing agent. The present application also provides a hair dyeing process using the indigo darkener. According to the present invention, multi-layer scales on the surface of the hair are opened at large angles under an alkaline condition and/or high-temperature condition, so that large indigo dyeing particles would be easily absorbed in the scales； a black indigo oxocarbon or carbon black is generated under the action of the alkaline dyeing assistant and the oxidizing agent at a high temperature by using 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 the hair are closed under an acidic condition, so that the indigo carbide is firmly clamped and fixed in the scales of the hair； finally, the blue indigo dyes white hair into black hair, and the chromaticity, dyeing fastness and glossiness are comparable to those of a chemical hair dye.La présente invention concerne un agent indigo pour foncer les cheveux, contenant : 0,1 à 99 parties en poids d'un colorant indigo, 0 à 99 parties en poids d'un auxiliaire de coloration alcalin, 0,01 à 99 parties en poids d'un auxiliaire de coloration acide, et 0 à 99 parties en poids d'un agent oxydant. La présente invention concerne également un procédé de coloration capillaire utilisant l'agent indigo pour foncer les cheveux. Selon la présente invention, des écailles multi-couches sur la surface des cheveux sont ouvertes selon de grands angles dans une condition alcaline et/ou une condition à haute température, de telle sorte que de grandes particules de coloration indigo seraient facilement absorbées dans les écailles ； un oxyde de carbone indigo noir ou du noir de carb
