In Peru, cotton textiles have been found that are the earliest evidence for indigo dyeing to date - about six thousand years old. Both the LA Times and the Smithsonian Magazine have an article about these finds.

It really is an amazing find - firstly because these textiles have survived at all for six thousand years, in a condition good enough to still see the blue colour, even though it's faded, with the naked eye. Secondly, it's not just a few fragments, it's thousands of them. Thousands!

The fragments have been cleaned and partly analysed; you can read an article about the dye analysis here.