It is important to understand the "after-effects" of unblocking an account:

Unblocking someone does not automatically make you follow them. If you want to see their tweets in your feed, you must hit the Follow button immediately after unblocking.

Unless your account is set to "Private," the unblocked user can now see your tweets, media, and replies.

You will see a list of every account you’ve blocked. Tap the next to the user’s name.

Click the (three dots) icon on the left-hand navigation sidebar. Select Settings and privacy . Click on Privacy and safety in the center column. Select Mute and block , then click Blocked accounts .