When Blackburn fell out of the Blackhawk you see the main soldier put on his goggles and start to slide down the rope. If you look behind him another soldier is about to slide down but when the camera changes the soldier disappears.
Actually the "other soldier" is a member of the flight crew. He isn't about to head down the rope, he's securing so that they helo can get moving as soon as possible after the last Ranger is on the ground.