And the stars of heaven fell unto the earth, even as a fig tree casteth her untimely figs, when she is shaken of a mighty wind.
If a wandering Black Hole whizzes past the Oort Cloud this may actually happen, or perhaps I should say "come to pass". I don't suppose that there would be much or any warning and certainly no defence against it.
As I have not yet mastered "Jaunting"*, the only option would be to fly continuously around the world staying on the side that was not being bombarded. Refuelling might be a problem.
I seem to have woken in a pessimistic mood today :(
*Although the return trip from the pub last night appears to have been instantaneous.