Open isn't as some expect it to be then - as the block feature and delayed menu exit have been features of the game for as long as the game has been released - and block has only ever been strengthened and made easier to use over time.
Players get to pick and choose who they instance with in this game, whether through mode choice or use of the block list to excise specific players from instancing with the player in either of the multi-player game modes - and their decision precedes and may over-ride the desire of any other player who wishes to play with them.
The block list circumvents the intended function of the mode by cretaing adhoc "modes" within them. The fact that it's designed that way is not disputed. I'm saying that it's a stupid design that shouldn't exist that way - regardless of how long ago it's behavior has had this effect.
The modes are intended to be the means of control over who players instance with. If you give that power to individual players via a separate function (as fdev has) then you've undermined the very existence of the different modes. Currently with the way the block list works, if i have a group game where players in my group must play a certain way or they get booted from the group, players can use the block list to effectively create a group within the group. invisible to all they wish to block but not to their buddies. This occurs in open to a much more realistic scenario where open is no longer open but effectively turned into a group mode where that group varies for every player.
If you want a group mode where players can't hurt other players ...just have Fdev create that mode and be done with it. It makes no sense to undermine open more than the networking system does. Open should mean any and all - whether you like it or not. That's what open exists for.
The only way i'd see a justification for the current way the block list behaves with instancing is if the instancing part of the block list was limited to 5 players max to take effect after any current shared instance with who you are adding. That should be enough for any exceptional situations without abusing it as a way to create your own mode.