Yes, it is absolutely possible to unblock a blocked user or channel if you decide on so. To do so, follow the steps below:
Step 1: Go to YouTube Creator Studio.
Step 2: Select Settings from the menu on the left-hand side.
Step 3: Further, select Community.
Step 4: You will find the list of users' profiles added in the Add Hidden Users box under the Automated Filters.
Step 5: If you would like to reinstate a user's commenting privileges, you can select Remove or the cross sign next to their name.