It might only be blue because we know the word "Blue." In africa there is a tribe that doesn't have a word for blue, but many words for green. When experimenters showed them pictures of multiple hues of green and on square of blue.
Until they learned a word for blue, they did not see it.
In ancient text, the word blue is not mentioned. As in the old Hebrew Bible and Homer's the Iliad.
Listen to this podcast. It's very interesting.
http://www.radiolab.org/2012/may/21/sky-isnt-blue/
