Why are jeans blue?

"Denim is unique in it's singular connection with one colour. the warp yarn is traditionally dyed with the blue pigment obtained from indigo dye. Until the introduction of synthetic dyes, at the end of the 19th century, indigo was the most significant natural dye known to mankind,linked with practical (sic) fabrics and work clothing. the durability of indigo as a colour and it's darkness of tone made it a good choice, when frequent washing was not possible. In 1870 BASF in Germany, originally suppliers of natural indigo had started the search for a synthetic substitute, in 1894 the process was perfected." - Designboom.com