I seem to recall setting the Difficulty at John Silver, which makes sense - save file has "difficulty: 1"
I'm afraid I have no idea who Black Dragon is or what warning was sent my way. I've encountered a White Dragon who I've bought some tokens from.
EDIT: reading the save file, apparently I'm 23 months in, not 18 like I was thinking.
Two Flak cannons seem sufficient to stop the missile, assuming they both hit.