To block effectively you have to beat the ball to the spot. Only by being there first can you be in a good enough mechanical position to control the ball.
I've seen lots of high school catchers who move to the ball almost as if in a panic, lunging at balls in order to block them. I'm not sure if they do this because they are not physically quick enough to get to the spot, or because they don't anticipate well enough, or perhaps because they don't know their pitcher's tendencies (such as how their curve ball breaks.)
The blocking motion should be under control and smooth, but to do that requires quickness. I think most people have little idea as to the quickness and overall athleticism required to block effectively.
Mechanics are very important, to be sure, because without proper body position the blocked ball won't stay in front. But don't forget to work on quickness (especially lateral) and anticipation, because you HAVE to beat the ball to the spot.