The reason for this modification was based on other people’s request because they wanted to set a Blackboard reference there, but the way it was before (IBlackboard) it was not possible to drag and drop a Blackboard gameobject in the field (that is basically Unity Inspectors fault).
On the other hand, what you say is true.
Ideally, I want it to be IBlackboard of course, but the issue with Unity inspector in regards to interfaces was problematic. I will have to come up with some inspector hack to support both as it seems. For the meantime, you can simply open up GraphOwnerInspector and at line #295, please change the type from “Blackboard” back to “IBlackboard”.