Indigo Powder combined with henna powder, an abundantly used natural hair dye combination to get shiny black hair has been used since almost 4000 years ago when no artificial hair color was even thought to be invented. Indigo powder is actually grounded leaves of the plant famous as Indigofera Tinctoria . It is named as the indigo powder because it has the deep blue pigment in it. Although indigo powder has blue pigment it only gives dark brown/black hair color when mixed with henna and not blue color. During the 19th and 20th century indigo was such a common hair dye in Europe and the US, that it was marketed as "black henna". Henna is never black, henna never dyes hair black, but powdered indigo resembles powdered henna and it does dye hair black. So, when indigo was sold as hair dye, it was called "black henna”.
Make sure the indigo powder you are using is green in color and smells like dried up peas, when mixed with water it gives ink blue coloured paste.