Observed by 4.7 preview 5 user. After closed investigation it turned out he had two consequent AIController::UseBlackboard calls, which triggered a ensure failure in AISystem::UnregisterBlackboardComponent. After a brief investigation I've found that AISystem::RegisterBlackboardComponent is not being called in all cases it should.
There's no existing public thread on this issue, so head over to Questions & Answers just mention UE-9287 in the post.