What happened:

I ran the following command "/scoreboard objectives add Blocked minecraft.used:minecraft.shield" and the scoreboard objective was created successfully. Then I ran the second command "/scoreboard objectives setdisplay sidebar Blocked" so that I could see when my score increases.

Then I tried to block all kinds of things (including skeleton arrows, blaze fireballs and zombie melee attacks) with shield in my offhand slot and the scoreboard score didn't increase.