Indigo Powder combined with henna powder, an abundantly used natural hair dye combination to get the lovely and 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.
(Indigo powder itself does not work effectively on grey hair, it has to be applied either by mixing with pure henna or after applying pure henna (2 step method) on your hair)