The block number on your shield is your block value. Strength also only adds to block value.
Block value only raises the amount of damage that blocking removes. For example if you are going to get hit for 50 damage, you block it, and your block value is 20, then you will only get hit for 30 damage instead (20 blocked).
There are only 2 ways to increase your block percentage One is with Defense Rating, which increases your dodge, parry, block, and the chance of enemies missing you. The other is with Block Rating, which only increases your block %age, but it will increase it much faster than Defense Rating.
I hope that answered your question.