Why Is Denim Blue?

Denim is unique in it's singular connection with one colour. 
The warp yarn (that running the length of the fabric) 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 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.

Birgit Lohmann  Designboom, 2000