This is a simple one. On the website, login then go to your own profile. You’ll see an option in the left column to view Blocked Members. In the pop-up, select whom you’d like to unblock.
On the mobile app, tap on Browse, then tap Blocked Members.
Keep in mind that once you block someone, there’s a 72-hour waiting period before you can unblock them (this is to prevent abuse of the block feature).
Hope this helps,
The Grokio Team