Me and my friend don't remember and we're arguing over it. END IT.

example of what blocking? i just played this recently

There wasn't a "block button", but I'm pretty sure that if you had the shield you would block automatically as long as it was equiped, you were facing the right direction and you weren't attacking.
 
Not 100% on that tho.

Yes block as in defend

yeah there was blocking with the shield.

You don't push any buttons to block. If an arrow was headed towards you, all you would do is face it without attacking and you would block it.  Your shield is automatically equipped as soon as you get it.  

There was no block button but you blocked automatically if you weren't attacking.

Agree with above, also, some enemies would block your sword attacks (i.e. the soldiers at Hyrule castle.)

If the Mirror Shield doesn't define blocking, then I don't know what does.

yes you can. I did it often to defend against arrows.

" There wasn't a "block button", but I'm pretty sure that if you had the shield you would block automatically as long as it was equiped, you were facing the right direction and you weren't attacking.  Not 100% on that tho. "

Yeah, you just gotta make sure your shield is lined up properly. There was no block button, though.

From my recollection, the shield only blocked projectiles, not melee attacks i.e. the soldier's swords (those needed to be parried with your own sword).  Also, the higher level of the shield, the more attacks it could block.  The small blue shield could block rocks and arrows but not fireballs.  The red shield could block fireballs but not laser beams.  The mirror shield could block almost all projectile attacks (but not boss strength projectiles like lightning or Ganon's fire bats).
 
Also note, you can "hold your shield out to the side" while you hold your sword in front of you.  However, there is no way to hold your sword out and hold your shield down to protect from attacks from the bottom of the screen - due to Link changing his shield hand from right to left because of a sprite flip.  In other words, if you face left or right and hold your sword out, your shield will always be facing the top of the screen.
 
When I was younger, I think I read in some gaming magazine that it is because "Link shields himself from the evil of Death Mountain," - Death Mountain being at the very top of the world map.  Of course, this is inaccurate because if Link faces up or down and holds his sword out, his shield will be on the left or right, respectively.