If you can stomach watching it, here's NFL's Anatomy of a Play, breaking down the pic and return.
http://www.nfl.com/videos?videoId=09000d5d80e97cd9 . Cosell actually points to that particular block and commends it. Because they are using league cameras for these, you can get much different angles and views. One thing I noticed was that the ref was 12 yards downfield at the time of the block, and at the exact timing of the block, was focused on getting around two Steelers on the ground. He couldn't have seen the block to make a decision one way or the other.