I don't know the answer to your question, but I can share a recent experience.

I flew Economy on an ORD-LAX (B738) flight ... the friendly gate agent told me she had blocked the seat next to me, but the person sitting next to the window invited somebody to sit next to him. I politely protested to the FA (all that was left was a couple middle seats in the very back) to no avail.

My point is, even if "seat blocking" is offered, will it really matter in the end?