Indigo does not dye hair black on its own because it does not attach to hair. You will have to use it with Henna because indigo attaches to henna.
Indigo powder is from the ground leaves of the indigo plant (Indigofera tinctoria) which releases a deep blue dye used for centuries to dye textiles and hair. Indigo combined with a henna application on hair produces deep rich red/brown shades through to a rich jet black colour. This is considered safe alternative for people who are sensitive to chemical dyes. A doctor should be consulted if you have any concerns.
Things to note about Indigo