Woad

https://en.wikipedia.org/wiki/Isatis_tinctoria

Isatis tinctoria, also called woad (/ˈwoʊd/), dyer’s woad, or glastum, is a flowering plant in the family Brassicaceae. It is occasionally known as Asp of Jerusalem. Woad is also the name of a blue dye produced from the leaves[2] of the plant. Woad is native to the steppe and desert zones of the Caucasus, Central Asia to eastern Siberia and Western Asia (per Hegi[3]) but is now also found in southeastern and Central Europe and western North America.

Since ancient times, woad was an important source of blue dye and was cultivated throughout Europe, especially in Western and Southern Europe. In medieval times there were important woad-growing regions in England, Germany and France. Towns such as Toulouse became prosperous from the woad trade. Woad was eventually replaced by the more colorfast Indigofera tinctoria and, in the early 20th century, both woad and Indigofera tinctoria were replaced by synthetic blue dyes. Woad has been used in traditional Chinese medicine for centuries. There has also been some revival of the use of woad for craft purposes.