Indigofera tinctoria is a shrub belonging to the bean family. Nobody knows exactly where it's from originally but its leaves have long been the source for blue dye in India and made its way to Europe a couple of millennia ago. Farther north the same pigment, although much less concentrated, used to be extracted from the woad plant by fermenting it in urine, preferably from drunken men. That was once the color of blue jeans, the Swedish flag and the soldiers' uniforms.
However, Indigo pigment is fugitive and all paint manufacturers have replaced it with synthetic, lightfast dyes. For example Winsor & Newton blends Lamp Black (soot) with Phthalocyanine Blue (aerugo) and the violet Beta Quinacridone. Daniel Smith makes it with Indanthrone Blue and Lamp Black, while Lukas has it Benzimidazolone Carmine, Phthalocyanine Blue and Lamp Black.
In other words: real Indigo is another pigment no longer among us in the world of watercolor, although it is still available for dyeing hair and yarn. As my followers may know, I stick to single pigments but once in a while I take out some of the convenience colors including Winsor & Newton's composite Indigo, which I find very beautiful.