Through sheer luck, a planet-hunting satellite has detected micro-lensing events that are eventually deduced to be due to a primordial black hole (one that masses less than our Sun). The hole is quiescent at discovery, because it's been in the low-density interstellar medium for millennia, but it's soon discovered that a) its heading relative to the galactic background is constant, and b) its distance (detected by additional lensing events) is reducing.
Anyone familiar with navigation knows that "constant bearing, closing range" is a more precise way of saying "collision course."
A tiny black hole passing through the Solar System would be disastrous, but not an immediate extinction event -- disrupting the orbits of the planets wouldn't kill everyone quickly, though there's reason to believe it would do so over a period of centuries (you think we have a climate change problem). However, every refinement of the measurements continues to indicate collision course -- in fact, if not a direct hit collision with the Sun, a miss so close that the black hole will draw gas from the Sun as it passes.
The very best possible outcome is a series of flares that will each sterilize the face of the Earth then in day, down to a depth of multiple meters in water and a few tens of centimeters in soil. There's enough error to believe there's also a possibility of a black hole induced supernova explosion (if the hole center-punches the Sun); such an event would melt the day side surfaces of the inner planets, sweep off their atmospheres -- well, further details are counter productive.
The good news is, we have a bit more than a century before the hole arrives. The bad news is that our message in a bottle needs to be well beyond the Kuiper Belt by that time, to be sure a supernova doesn't disrupt its systems enough to effectively destroy it.