Is it appropriate to say "Baruch Dayan HaEmeth" when you hear of sad news about a person you don't know, or a person you have corresponded with but haven't met?
Does it make a difference if you've heard a great deal about the person or if the first time you're hearing of them is the report of the news?
Does it make a difference if you've gained from the person (knowledge, business, entertainment, etc.) despite not knowing them or corresponding with them (read their book, seen their work, studied their expertise, etc.)?