In my own personal experience, I've had both domestic and international blocked bulkhead seats open up at, right after, or some time after t-24 checkin. I got really good bulkhead seats on my non-upgraded flights today, which is awesome because I would have been stuck in middle seats.

As always, YMMV. Sometimes they are blocked up until the GA assigns them to any potential disabled passengers, or those awaiting a seat assignment. If they don't open up before the flight, see if you can get to the gate and talk to the agent(s), as others have suggested.