[jacobolus]

I have known people who lived in a run-down studio apartment or just a bedroom in a shared house who had a sizable blackboard or whiteboard. So this is clearly more of a personal choice about your space than anything inherent about the tool. But if you are by yourself or with 1–2 colleagues (not trying to explain to a group) you can certainly do the same work on paper. Reasoning through a problem on paper becomes unworkable when you try to show more than a few other people at a time, which is where the blackboard comes in.