The Taekwondo upper block is known as Chukyo Marki in our style and Age Uke in Karate.
We were practicing Chukyo marki today as a line drill.
Today in the line drill moving forward in a lunge stance, we incorporated a backward glance. This is done when your feet come together and when your arms fold - you glance either to your side or back before looking forward again and then performing the Upper Block.
The next stage, we practiced raising the shoulder of the blocking hand and covering up with the back hand as we move forward - so essentially covering up the side of the head (on the side of the block). During the fold, the body naturally compresses, decreasing target size. This is a good cover as your opponent will try to strike you from the front - raising your hands in front of you, then trying to come over your shoulder and knocking you out from the side of the temple. The fold for the rising block nicely protects the side of the head.
Many people mistake the Taekwondo upper block as a block purely against something coming straight ahead and from the top of your head. Many people fail to understand that you need to 'apply' the block based on the nature of the attack. Just attempting to do the upper block will result in you failing to adequately intercept a downward or sideward swinging motion to you.
Keys for success are to make sure the upper block can rotate horizontally and cover the side of your head equally as well as covering the front and top of your head. Also you need to study the flight path of the block and force yourself to use this upward motion plus a drop down of your head to quickly intercept whatever downward swing is coming your way.
Check out E-How: Doing an Upper Block