What historical evidence exists for the origin of the association of blue colors with boys, and pink colors with girls?
I realize this is a cultural specific trait. An article "Beyond Pink and Blue" looks at how color preferences vary across cultures, but this question is specifically about the cultural norm that a boy baby should have a blue blanket.
What is the earliest known reference to this association? Or, alternatively, is there any evidence that this association goes back as far as recorded history?