You now have a couple of statistical methods for rating your blockers. I suggest you try one or the other during a team scrimmage.
For me, blocking is one of the most difficult skills to teach and assess. We all know about “penetration” and how important that is to successful blocking. However, I would contend there are several other key factors that may be even more critical. They include setting the block, hand and arm positioning, vision and timing. And when it comes to evaluating a player’s effectiveness at this skill, we are usually left with “blocks per game” as the only answer. (By the way, wouldn’t “blocks per attempt” be more informative?)
The light first went on for me when I was in China some 30 years ago. The USA women’s national team was competing in a tournament, and I had the assignment of scouting Russia. As always, I settled myself in the stands behind the end line. (This location is used because it gives the best view of serve-receive formations, attack patterns and blocking schemes.)