Considering the Behavior Tree as a brain, we can think of a Blackboard as its memory—more specifically, the memory of the AI. The Blackboard stores (and sets) key values for the Behavior Tree to use.
They are called blackboards because, in a classroom, the blackboard is a place where a lot of information is conveyed, but most of it is shared among students; the single notes that are given out to students are personal. You can imagine the students as the different tasks (and nodes) of the Behavior Tree, and the Blackboard as a shared place for data.
Blackboards are fairly simple to understand, since they are only a little more complex than a data structure. The only difference lies in the possibility to assign to a Behavior Tree to a particular Blackboard, which is shared by every node of the tree. As a result, each node...