How to ferment green indigo powder to make it blue
Try mixing the unfermented indigo powder with water in a bowl, and 'beating' it, using a smaller bowl to scoop it up and pouring it back until the mix turns blue.

Consider adding lye [NaOH] or slaked lime [Ca(OH)2] to help the fermentation process. The effectiveness might depend on how the unfermented powder was produced, though.