Indigo

The leaves of the indigo plant are a major source of natural indigo, a blue dye once used for denim jeans, but now largely replaced by synthetic dyes. Indigo is still grown in some parts of South Asia, and synthetic indigo is widely used for traditional textiles.
Watercolour of an indigo plant with flowers, 1826-1833.
Mad dogs...
In some parts of India the plant used to be well known for treating rabies.
Indigo and Gandhi
Workers on indigo plantations played a key role in British India's independence movement. Gandhi led the battle for their rights and was soon supported by the poor people of smaller towns and villages, many of whom worked in indigo factories.
The woad connection
Consumers are now favouring natural dyes again so fields of dye crops are gradually returning. Aside from indigo species, several other plants also produce blue indigo dyes. Perhaps the best known is woad, once the major blue dye of Europe.
Blue jeans
Denim jeans are dyed with indigo, today from synthetic sources.