As a person who wears jeans almost every day and has for most of my life (excluding my college years when it was only leggings...I still have legging days), this was really interesting!!
Obviously, they're blue because of the dye, but the indigo dye was chosen because it attaches to the surface of the cotton instead of penetrating it. That's why blue jeans get their signature faded look! It goes a lot deeper than that too so click the link below to watch the video!