ah yes the mental block.
Not too long ago I made a post here that I think was a good example of the mental block
I had been playing an online game for months. Every time I made a set of three, the score would pop up right over the game and then scroll up. The odd thing was that it took me all those months to actually "see" the scores scrolling up. There they were, covering the game, impeding my view, but for all those months my brain refused to see them.
I think a JW's mental block is very similar. A person can be so focused on one aspect of something he doesn't even see what is right in front of him. The WTS is great at using this ability to prevent people from seeing what is right in front of them even when it is in black and white on the page they are reading.