The seat is "blocked" on the computer, in that no one can pick it when buying a seat by phone or on-line. But remember the gate agents have enormous discretion when filling the plane. The bulkhead middle seat, or row 9 on the 757 for example, is a popular choice over a middle in the back, and when someone at the gate pleads for a seat with more leg room because of a "medical condition", or some other excuse, the gate agent can and will give it away. At the gate the only "blocked" seats are ones that have been purchased.