The long-forgotten natural blue dye was called "folium."
An amazing restoration of medieval knowledge by these scientists in Portugal. The study itself: https://www.science.org/doi/10.1126/sciadv.aaz7772
The plant in question (Folium, aka Chrozophoridin): https://powo.science.kew.org/taxon/urn:lsid:ipni.org:names:340969-1












