Generally unblocking a hand can be seen as the opposite of blocking a hand. For instance, if you hold the Jh on a 3 hearts board by the river you do block quite a serious amount of villain's flushes (e.g. JhTh, Jh9h etc...). As I understand until here you're all fine. Now to finish this very simple example consider your holding not including any hearts. In this case you unblock your opponent's value range (flushes in this case). Meaning because you don't have any hearts it's not only neutral but even slightly more likely that villain holds a flush (compared to a theoretical situation where you wouldn't have cards at all). This is because you already know he doesn't have any of the two non heart cards that you are holding. If you weren't in the game he could have those cards and if you compare the two likelihoods of him having a suited heart hand it's higher in the example where you hold the two non-heart cards (or in other words where you UNblock his hearts)

As you might already know both blocking and unblocking become much more relevant when ranges run tighter. One of these situations where ranges run tight could be an extreme polarized situation on a non-flush board where there is only one possible straight and you suspect that villain would only play this way for value with a straight or sets. Lets take a board of 6 7 T K 2 . Maybe from preflop action you can disregard TT+ so that his only value range would be 98 for the straight and 66,77. If you in this case block (i.e. hold) a 6/7/8/9 this suddenly blocks a considerable percentage of villain's value holdings. In scenarios with much wider ranges blockers can still be helpful but are often simply used to find a split for calling/folding among hands that are all very similar in value regarding bluffcatching. In a case like this you of course would still go with the hand that has the blocker over the hand that doesn't even if it might only block 2-3% of villain's value range.

For unblocking it's very similar. The tighter the ranges the more sense does it make to start thinking about which hands you unblock with your holding. A common example for unblocking would be a board of A T 7 8 3 . If you hold KQ here you UNBLOCK villain's value range (e.g. J9, AT, A8s, A7s, A3s, sets) but at the same time you block some of his bluffs (KQ/KJ/QJ). Note that it's not always crystal clear into one direction. This very example might be flawless if you assume villain only value bets 2 pair +. But if he would also valuebet AK/AQ you once again block some fraction of his value range.
However, if in the same example you hold 65s you now suddenly UNBLOCK some of villain's good bluffs (the before mentioned KQ/KJ/QJ). So by not holding any King, Queen or Jack you now make it more likely villain is bluffing because he could choose his main bluffs for a 3barrel from these hands and he will hold these hands more often if you don't block i.e. unblock them.

Ok...so unblocking is just another way of saying we don’t block. The thing that seems strange is using the term unblocking that implies that they were previously blocked and now we no longer block them. I guess that’s where I was confused. Thanks a lot for great explanation