I think "blocking" is a discretional thing. It has never been a policy. Blocking is up to the check-in agent/flight control manager. If a flight is super empty, they will gladly let you block. If it is 90% above they are unlikely to block it for you as they need the seat. Sometimes, they can block but it is never guaranteed until aircraft has literally pushed back.

Over the years I have had CX block me a seat in Y on HKG-YYZ, HKG-LHR, and HKG-KUL.

I have found though, that outport staff are far less likely to be able to honour a blocking of a seat. I think this is because flights are still managed from HKG, and so they cannot liaise with a flight control manager as easily and ultimately have less control over the flight. In HKG, if the check-in agent is committed to trying to help me block a seat, they always call a flight controller.