Could planet x be a black hole?

I am going to assume you are referring to the recently hypothesized planet 9.

"It's purely theoretical at this point. Batygin and Brown predict its existence based on unusual perturbations of the solar system that aren't otherwise easily explained."

"Based on Batygin and Brown's previous calculations, the hypothesized planet weighs five to 10 times as much as Earth and is, on average, 20 times farther from the sun than Neptune. It takes Planet Nine 10,000 to 20,000 years to orbit the sun."

Lets assume it has its maximum mass of 10 times the mass of the earth. Now lets calculate how big this black hole would be by calculating its Schwarzschild radius `R_s` . This is the radius of a spherical blackhole to its event horizon.

`R_s=(2MG)/c^2 `

`M` is the mass of the blackhole, `G` is the gravitational constant and `c` is the speed of light.

Plugging in 10 times the mass of the earth for `M` gives `R_s=90 mm` . 90 millimeters is extremely small and would classify this object as a primordial black hole. 

"A primordial black hole is a hypothetical type of black hole that is formed not by the gravitational collapse of a large star but by the extreme density of matter present during the universe's early expansion."

So the answer to your question is yes it is possible, but perhaps unlikely.