Blue was the chosen color for denim because of the chemical properties of blue dye. Most dyes will permeate fabric in hot temperatures, making the color stick. The natural indigo dye used in the first jeans, on the other hand, would stick only to the outside of the threads, according to Slate.

Why is denim normally blue?

Why most jeans are blue

People have been wearing blue jeans for centuries. Originally, the blue color came from a natural indigo dye. The dye was chosen for the way it interacted with cotton. When heated, most dyes penetrate the cotton fibers but indigo dye attaches to the fiber's surface, instead.

When did jeans become blue?

May 20, 1873 marked an historic day: the birth of the blue jean. It was on that day that Levi Strauss and Jacob Davis obtained a U.S. patent on the process of putting rivets in men's work pants for the very first time.