You will see your list of blocked accounts. Tap Unblock next to the user’s name.
Click on your Profile Picture in the top-right corner. unblock youtube user
Under the "Privacy" section, tap Hidden users (or "Blocked users"). You will see your list of blocked accounts
Tap Your data in YouTube (this is where Google hides advanced settings). unblock youtube user
Click the Unblock button (usually a blue link or a person icon with a minus sign) next to their name.
You will see a list of everyone you have blocked. Find the person you want to unblock.