Well, "indigo" at its darkest is the same as "navy blue", which is the darkest shade of indigo dye. It is mostly seen as a shade of blue, just as peacock is. Neither is normally treated as one of the basic colours.

However, when people are taught the seven colours of the spectrum, one of which is the unfamiliar "indigo", they're pretty well bound to hunt for a meaning for the word. From the order of the spectrum, "indigo" has to be between BLUE and VIOLET. So, if you take BLUE to be BLUE, then it follows that "indigo" has to be some shade of blue-purple.

It would have been so much simpler if Newton had used the same colour terms as people do today!