It's my impression (a pure speculation by watching how free seats distribute) that the blocked seat is accessible to other EBG/EBD members. So if you seat in front of the Y cabin, it is quite likely that one of them will sit next to you, to be in the front. (besides this, I think airport agents also seem to make random overwrites - pure guessing). I think I had best luck with seat blocking with selecting seats a few rows after the "curtain". Not sure if this is just my impression, or is there any hard knowledge behind it.