When did jeans become blue?

On May 20, 1873, San Francisco businessman Levi Strauss and Reno, Nevada, tailor Jacob Davis are given a patent to create work pants reinforced with metal rivets, marking the birth of one of the world's most famous garments: blue jeans.

What is the original Colour of denim?

Denim was originally dyed with indigo dye extracted from plants, often from the genus Indigofera. In South Asia, indigo dye was extracted from the dried and fermented leaves of Indigofera tinctoria; this is the plant that is now known as "true indigo" or "natural indigo".