I don't think they are blocked I had 6H on a recent ORD-LHR flight. I had also read that they were the preferred seats and when I booked the flight neither were available. I kept checking back periodically, and then 3 days before the flight both 6CH opened up. I assumed that whoever held the seats had either switched flights or been upgraded to 1st class. In any case, I preferred 7A which I got for the return, I felt that the middle seats have a little two much open public area with people passing by in the aisles.