There's a difference between a scientific guess and a scientific explanation. The latter is only possible with enough evidence to support it.
In the case of this Nightfall asteroid on a trajectory to Earth ... Ann has brought up many clever points, truths and facts about it.
While there's nothing wrong in theory with black holes diverting objects from their paths, the attractive force of said black hole would need to be so large that it couldn't have gone undetected in our solar system. Today, scientists are for example studying invisible matter in the center of the milky way that causes whole galaxies to slingshot away from their expected orbits, thus proving the existence of a giant black hole. The natural force that needs to be present in our solar system would have been discovered (or not discovered but been expected) probably a century ago.
While physically a bit unrealistic it's entirely possible that something crashes into Nightfall our astronomers were previously not aware of. Take a look at
this article for example - it talks about a watch system of potential threats to Earth and it's far from being complete even in today's time. Go back a few years before movies like Armageddon or Deep Impact aired and you will find a society that's even less aware of deadly impacts from "out there". When Nightfall can surprise the world such a short time before impact, a collision is at least a remote possibility.
