I'm not sure what you mean about Indigo girl but I am familiar with the word Indigo itself.
Usually people will said that Indigo is someone who have extra ability beyond other people usually have, in feeling, seeing or hearing. The word Indigo have connection to the supernatural phenomenon experiencing by someone in wide area of phenomenon, and people will correlated it with reincarnation or stuff like that, but in my own term it is not what indigo is.
In my opinion, indigo is high sensitivity toward spiritual world which make the person who have it become more aware on vary spiritual activity around us, which is not common for people to be aware of it.
The Indigo Girls are an American folk rock duo, consisting of Amy Ray and Emily Saliers. But what they represent is perhaps more important than their work. Both are self-proclaimed lesbians. They champion the causes of lesbian rights, the environment, and rights of the American Indian among other things. They are considered to independent and forward-thinking somewhat reminiscent of the youth of the 1960s. So an Indigo girl is someone who is free-thinking, independent, and champions the causes of the underdog. There might be other definitions but this is the only one I'm aware of.