Sample identified as pure indigo powder from source.
Note:  Indigo is often sold as "black henna", and in powder form looks very similar to henna.  If you add water to indigo, in about 20 minutes, you'll see blue dye form where there is contact with air.  You can see the blue dye oxidizing at the margin of this sample.

If you have a sample of unidentified henna powder and want to determine whether it is indigo, henna, or a mix of henna and black PPD, add water to each.  In 20 minutes, the henna will glaze over browninsh, the PPD mix will exude black, and the indigo will glaze over dark blue.