In all of the chapters it's mentioned that Coldhands face is covered with a scarf that covers his mouth. I don't remember what Benjen's original eye color was, but I'd guess not blue. That and the fact that he appears to be dead, may have made Bran avoid looking too close. Why Benjen wouldn't reveal himself to Bran is a decent question, but it may be that he lost some or most of his memories after being turned.