The best way to learn how to block effectively is in the tutorial within the game, however here are a few tips.
Don't panic block (just block when a enemy runs at you) this will cause the enemy to get a strike in as timing is everything. Your block animation need to have your weapon held up in front of you for the block to work. Using a shield gets rid of this issue due to just being able to hold the shield up.
To make sure the block works, you will need the tip of the enemies weapon in view, such as a overhead you will need to look slightly up to make sure you get the block in, slashes you need to look slightly to the side. If using a shield, you just need the shield in between where the weapon is going to hit you and their weapon.
Finally, it is just a lot easier to use a shield, as you don't have to worry about timing so much and just hold up the shield to block the incoming attack.
Just get 100 successful blocks and this achievement is yours.
Any suggestions for this solution just comment. :D