Indigo is a natural dye, the same used for dying blue jeans. It can make your hair blue-black, if you already have dark hair. Indigo is messy, but is safe. Most black hair dyes contain ppd, a dangerous chemical that can cause allergic reactions. This helps give the indigo a base to stick to. You can mix indigo into henna to get shades of reddish-brown to auburn to brownish black.