In my mind it’s all about intention. If you place a beacon next to the north of me (cardinal direction chosen randomly) and I’m only expanding south right now, then I wouldn’t say you’re blocking. But if you see a bunch of unfinished builds and progress being made on things to my south and decide to cozy up not just a few plots but along my entire southern border, I’d say this is blocking.
You’re right that there’s no clearly defined definition of “when X happens, that is blocking”. I’m merely offering what I think it is, which is based on 3 things, primarily.
1.) who was there first
2.) were plots intentionally left open for other Citizens
3.) do I (or any beacon owner) see this new neighbor as being someone who is blocking me from a project I’d like to do in the future?
I mean, there’s a reason the plot mode is activated when you hold a beacon in your hands. If you want to avoid any potential instances of blocking, just don’t build next to someone else unless you have their permission first.
Speaking of…I need to make a new suggestion thread.