It is that something just beyond our reach. Far away mountains have an Indigo cast that seems to cry out to the explorer in us all. This harmonic creates a sense of something beyond what is easily seen that has wonderment and mystery.
Indigo also symbolizes the falling of night after sunset when things are more difficult to see.
It can give a sense of cautious, spine tingling curiosity. The Central Nervous System in general is supported by Indigo harmonics. It is found over healthy nerve activity generally. Most natural remedies known for their calming effects emit Indigo harmonics, like valerian and chamomile. More information is available on this subject.