You sqeeze the plant until a purpleish blue die come out and then you put it in your mouth, swish it around, and spit it out onto a canvas to make a beautiful painting.
Indigo dye traditionally comes from the leaves of the indigo plant, which is a bushy legume. It has been used for dyeing for more than 4,000 years. The scientific name for one variety is Indigofera...
Although I'm not aware of any 'indigo' colored plants ('blue jean' or denim blue), there are some which produce naturally blue flowers: agapanthus, bachelor buttons, iris, corn flower, violets to...