Could Planet Nine be a tiny black hole or an exotic compact object such that it escaped detection? What would its bounds be?
I think the answer is included in my answer to Could a black hole pass quiescently through the Oort cloud? The mini black hole would be detected via its accretion from the interplanetary medium, probably at X-ray wavelengths.

A ten Earth mass black hole is a ten thousandth (or so) less massive than the Sun, but if it were in a distant orbit, it could be moving slower with respect to the accreting material than the 10 km/s I assumed in the answer cited above.

The net outcome is that this might still be an X-ray source well above the detection limits of all-sky surveys. What's more, it would move with time, so if followed up would show appreciable proper motion.

The variables here are the distance to the black hole and the speed at which is travels with respect to any medium it is travelling through.