UBlackboardComponent::InitializeBlackboard switched over to using referenced rather than pointers, and I missed this one while supplying transition API. Sorry!
Try this:
BlackboardComp->InitializeBlackboard(*BlackboardData);
Of course you need to make sure BlackboardData != nullptr before doing that.
BTW, why do you need to do blackboard setup manually? RunBehaviorTree will set up your BB for you.
Cheers,
–mieszko