I'm now saturated with learning about this color and it is intriguing to say the least. The original indigo comes from the indigo plant from, where else?, India and showed up in Europe probably as the result of both trade routes and the crusades - 12th century stuff.
I've found out that there are about 5 maybe 6 "indigos" with names attached but when you think of it we have a lot of reds and blues and pale and forest greens so why not. It is worth reading about and I'm knee deep in the Wiki footnotes and will be until dawn and I have to get some real work done...but I'll watch the night sky when it passes from black into violet and I can see nature's indigo.