Those had their own limitations. But, yes, today's equivalent--whiteboarding--can be an effective communication tool for someone who knows and understands material having a discussion with a (preferably small) audience.

That said, I remember lots of transcription from a professor's notes to a blackboard to students' notebooks with very little value-add along the way.

We always called them transparencies or "the overhead". I suppose the modern incarnation is the smartboard, which isn't too bad.