Removing black dye from the hair can be challenging for even the most experienced stylists. The first, and most important, step is to identify the type of dye that has been used. There are three main varieties of hair dye, and each type has its own method of removal. Aniline-derivative tints, used in most salons, are oxidative dyes which need to be mixed with a developer in order to work. Vegetable dyes, derived from plants, can be applied directly to the hair. Henna, for instance, is an example of a vegetable dye. Finally there are metallic tints, which are dyes derived from metals.
Professional hair dye, which is an aniline-derivative tint, comes in three forms: semi-permanent, demi-permanent, and permanent. While semi and demi-permanent colors will gradually fade from the hair, black can often be an exception. Any color, even a temporary rinse, can permanently stain the hair if the hair was blonde or porous before coloring.
If your hair was dyed black using a professional dye, it may be possible for the stylist to apply a color remover. Unlike bleach, a color remover won't lighten the natural melanin pigments in the hair; it will merely break up the color molecules of the hair dye. A color remover and a few washes with a clarifying shampoo can often remove black dye. If this doesn't work, it may be necessary to bleach the hair.
If you had colored your hair at home and it turned black, this situation requires special attention. Metallic dyes produce an exothermic reaction when in contact with bleach, and this can result in chemical burns to the scalp- as well as severely damaged hair. Unfortunately, many color removers will not remove this type of dye from the hair. Since bleach is not recommended, the best solution may be to simply let the black hair grow out. Hair that has been dyed cannot be dyed a lighter color, so applying a brown dye to hair that is stained black will have no effect.
Vegetable dyes are easier to remove from the hair than metallic dyes. Vegetable dyes work by coating the hair shaft with pigment. Unlike professional color, vegetable dyes do not penetrate into the cortex layer of the hair, which is the layer of the hair beneath the cuticle. Since vegetable dyes only coat the cuticle layer, each time the hair is colored another coat of pigment builds up on the hair's surface, making it harder to remove. When attempting to remove this type of dye from the hair, it's best to start with a harsh detergent (try making a paste with equal parts dish soap and baking soda). If washing the hair doesn't remove the color, it may be necessary to bleach the black out of the hair.