A planetary mass black hole orbiting a star would absorb all light which hit it, and it would have an extremely tiny diameter. Thus it would be effectively invisible and could be detected only by its gravity on other objects.
But even though the interplanetary medium in our solar system is a hard vacuum, it does contain a very thin scattering of subatomic particles, atoms, molecules, and dust particles. Thus some matter would fall into the planetary mass black hole. And possibly someone might care to calculate how much electromagnetic radiation at various wavelengths the infalling particles might emit as they are rapidly accelerated into the black hole and how detectable it would be.
A black hole with the mass of planet couldn't form by the collapse of a star which would have many times the mass of even the most massive possible Planet Nine.
So if planet Nine is a planetary mass black hole, it would have to be a primordial black hole, formed in the Big Bang which could have formed black holes with a wide range of masses. There are no known primordial black holes.
No doubt astronomers who searched for signs of them have calculated how a upper limit to how common they could be, and thus might be able to say how astronomically large the odds against Planet Nine being a primordial black hole are.