The rules basically say that you can't vent block, which everyone agrees with. Even that CAL folks agree with that one.
But then there seems to be confusion about the other two rules, which are that you CAN hallway block, but you can't lifeform block. This would be the difference between dropping a CC wall to cover the aliens getting into a room while you're setting up a siege base, and dropping a CC immediately in the path of a retreating onos.
Regardless, the rules on LM always have been that you use structures for their intended purpose. If you drop a CC you better plan on building it and putting IPs up around it. CC walls also have never been allowed. (that is the building walls ahead of time for protection when no alien are around) On the other hand, aliens blocking just about anything IS allowed, because the effort and res cost for aliens to block off an area with chambers is far more than that for marines. And I've always thought this was the intention of alien chambers, after all, a representation of them slowly taking over the map with infestation.
I'm pretty curious to see how the dev team builds in solutions for these problems. I'm betting building will ghost for a certain number of seconds before becoming solid, or will simply ghost until a marine actually starts building them. At that time we may possibly have a discussion on our blocking rules, but that doesn't mean anything will change, and that remains in the future.
THE MORE YOU KNOW!