I’m not familiar enough with blackboards but you should make your logic like this:
When an NPC enters the box trigger (Begin overlap), you must tell your blackboard that someone is already in the box trigger. It must be something similar to a bool that is set to true.
When it leaves the box trigger (End overlap), set this value to false and use this as a condition to move another NPC to that location.
As I said, I’m not familiar enough with blackboards, so maybe it’s missing something, but this is an overview of what you must achieve using AI.