let me clarify:
Perfect Blocks are available after the initial training from Captain Bernard. When you see the green icon, press the block button to fully defend against the attack, taking no damage and costing veeeeery little stamina.
Seeking further instruction from Captain Bernard, he can teach you Master Strikes, which are an enhanced version of Perfect Block that parries the opponent’s blow and does a quick strike, which opens up the enemy for a guaranteed attack (good for starting combos).
HOWEVER, to pull off a Master Strike you have to block when the enemy is starting their attack animation, BEFORE you see the green shield. At first this is very difficult, but my advice is to look at the enemy’s weapon arm, NOT the cursor in the middle of the screen. Once it moves back for an attack, press block.
Again, you need training before you can do this, otherwise you’ll just do a regular perfect block. To train effectively, I trained with a shortsword, no shield, meaning Cpt. Bernard was super fast and I had to get used to reading opponent attacks. After a few minutes, it became easy as can be.
Lastly, levels in Defence as well as the Weapon you are using increase the window during which you can successfully pull off a master strike, so the timing is more forgiving. I hope this clarification helps!
