One likely possibility is that unblocking you is a symbolic move. He's ready to move forward. This could mean moving forward in the sense of getting back together — but it's best to let him establish that. That's the other possibility: he's realized he wants you back.

What does it mean when a guy blocks you and then unblocks you?

When he's blocked you, he clearly doesn't want to talk to you. If he keeps alternating between blocking and unblocking you, it could simply mean he's struggling with his own feelings post-breakup.

Do guys ever unblock you?

Most exes will unblock you on their own accord so most of the time you actually won't have to do anything to get them to unblock you except be a little patient. Of course, there are also a lot of exes who wont unblock or contact you and it is these exes that might need a little extra push.