They absolutely block a lot of the seats to online selection until check-in starts. I had booked a ticket in February for travel in July. This was a flight with the QS seats and from the beginning there were only 6 seats available for selection. A couple of those got filled over the months and then when I went to do online check-in at T-48 basically all of the seats were available to select. Seen the same thing for 3 different trips.