The seat block is there for sure, not always, but it definitely is there, no question about it. Often you see it even in SH J if the load is light, they do not allow anyone to select the seat "next" to you. I have a flight tomorrow, for example, where A is blocked and I'm on C seat in SH. But, totally agree with intuition, it's something you can never take for granted but it sure happens (often).

To add, I think the block, when it happens, is only activated closer to departure, maybe when OLCI opens..