The Blackboard module adds a virtual blackboard to your project, allowing you to write data to it, which other objects can be notified about, or read the values of.
Essentially, the Blackboard allows objects to pass data into a central repository which other objects can then retrieve, without having to talk to each other. The Blackboard module will also notify objects that follow specific topic/subject values, or subscribe to events that are passed into the Blackboard.
This video demonstrates basic use of the Blackboard with the creation of the Blackboard Demo Scene.

Static reference to Blackboard is MainBlackboard

You are able to have multiple Blackboard objects in your scene. To help keep the required blackboard separated, the static reference to it is MainBlackboard.
The Blackboard prefab in the _Repository Prefabs folder has a MainBlackboard component on it, which creates the static reference to this objects Blackboard, which you can access with MainBlackboard.blackboard.