"Space is big. Really big. You just won't believe how vastly, hugely, mind-bogglingly big it is."
It seems like the night sky isn't so very big, so it should be easy to observe objects, right? Wrong. Once you use telescopes, the night sky becomes huge and if you don't know what you are searching for, you'll only find out about it by coincidence. The biggest problem is that the planets only reflect light, which makes them barely shine, so we don't really know where to look and most ordinary telescopes will be unable to detect this. This planet is so far from the sun that sunlight is not stronger than our moon, so the reflected light will be much less (see https://astronomy.stackexchange.com/questions/13282/how-bright-would-the-sun-appear-from-the-hypothetical-planet-nine-proposed-by-ca).
But let me quote directly:
If the planet happens to be close to its perihelion, Brown says, astronomers should be able to spot it in images captured by previous surveys. If it is in the most distant part of its orbit, the world's largest telescopes—such as the twin 10-meter telescopes at the W. M. Keck Observatory and the Subaru Telescope, all on Mauna Kea in Hawaii—will be needed to see it. If, however, Planet Nine is now located anywhere in between, many telescopes have a shot at finding it.
So part A) is too weak to form a good basis for the hypothesis.
Part B) is also not a good basis, because first of all, they are rumours (which could turn out to be false positives) and second of all, we have no idea what else can be said about them and where they maybe originated - because, well, they are only rumored to be observed.
This together is enough to dismiss your theory as highly unlikely.
Can we do more? First of all, we know the mass of the supposed ninth planet. It's not very big, just a couple of times the earth's mass. This means, it is far from sufficient to have originated from a star. This is the only known mechanism for the creation of black holes today, which means that from my limited knowledge, we are left with two possibilities: either this would be a small primordial black hole (if those exist) or a black hole that has nearly evaporated (if black holes evaporate). Both of these are hypothetical and we don't have an accepted theory for the formation of black holes of this size - let alone a single observation.
Second, we have not yet observed any solar system with a black hole not at its centre (recall that we have not observed small black holes), but we have already found a lot of systems with a lot of planets where nobody thinks they are black holes. However, our solar system seems special among these. This new planet would change this (once again from the authors):
"One of the most startling discoveries about other planetary systems has been that the most common type of planet out there has a mass between that of Earth and that of Neptune," says Batygin. "Until now, we've thought that the solar system was lacking in this most common type of planet. Maybe we're more normal after all."
This is even more evidence that this might just be a planet.