Indigo is a famous blue dye. The color originally comes from the Indian indigo plant. Indigo can now be produced artificially, but the color will be more intense and darker.
Indigo has remained famous over the years. We know it primarily from jeans.
The coloring, if done incorrectly, can look very detrimental and through various stages the coloring is difficult to acquire.
http://www.seilnacht.com/Lexikon/Indigo.htm
