I didn't watch the video but this is a common question/theme. "Blocking" when one doesn't know about blocking is usually "Active Blocking", aka riding at the front and trying to go at a slightly lower pace. "Blocking" in the real sense is "Passive Blocking" where you follow the ones pulling and not pull through.

There are very few instances where it's effective to Actively Block. Generally speaking it's dirty riding though, like hitting the front just before a hairpin and then going through it slowly.

There are even fewer instances where it's expected to either Actively Block or at least control the field in a physically blocking way. I forget which classic it is but one of them (Ghent Wevelgem?) has a narrow bit of road. Everyone hammers to get to that road and then whoever is at the front goes 4 wide and blocks the road. Then they eat, drink, recover, etc, before the road widens and the race resumes. It looks like it's sort of accepted by the field that if the team at the front decides to do this then it's the way it is. I watched whatever race a short time ago, Cervelo Test Team had 5 or so guys at the front in that narrow section. They even convinced a guy who managed to squirm his way through that he shouldn't attack. I think one CTT guy actually grabbed his jersey to keep him in place while they "discussed" things. The other rider eventually relented and took a spot just behind the CTT guys.

Passive blocking is virtually always accepted by all parties involved. It's the best way to do things. When Devolder won Flanders the second time Chavanel kept getting into breaks. He'd sit at the back and do no work for most of them. If his break companions insisted on dragging him to the finish he'd beat them because he was fresh. The others accepted this and worked hard even so.

Passive blocking requires the rider to have a very small sphere because the rider must be able to take wheels and defend his own spot. The rider must be aggressive and fresh enough to take wheels, respond to attacks, and chase down riders that actually get a gap. It can be very difficult to passively block but it's certainly easier than chasing.