Like, Kaz said above, block rating is like expertise rating, defense rating etc. It increases your chance of success in that area. Block value determines how much you actually block when you block an incoming attack.
Lets say you block an attack of 1000 damage. Normally you'd take the 1000 damage. If your block value was 500, you'd only take the last 500 damage, ignoring the first 500 that you blocked.
But unlike the other stats I mentioned, these do not push anything off the melee hit table. Just says how high a chance you have at blocking attacks, and how much you do block.
The box said: "Requires Windows 98 or better.", so I installed Linux.