If you are interested in black holes there are different career paths that you can take. In my opinion, Space Science and/or Planetary Science are not the best choices to study black holes as they may cover (if any) the theory and observations of BH only marginally. A best choice would be a career as a physicist, astronomer and/or astrophysicist.
As a physicist you'll have the mathematical tools and formalism to study the theory of blackhole radiation, the processes that creates and regulate BH growth and blackholes as a key component of galaxy formation.
As an astronomer you'll have the tools to study blackholes from an observational point of view while trying to match theory with observations. Blackholes radiate in a wide range of energies, from the radio to the gamma ray, therefore, indirect signatures of blackholes are suitable to observed with a wide range of observatories both on the ground and on space.
As an astrophysicist you will be between physics and astronomy, thus, an unique perspective into the theory and observations of black holes.
The main focus of my research is in super massive blackholes and how they can affect the evolution of galaxies. My university degrees are in Physics (BS, 2 x Ms and PhD) with an emphasis in astronomy/astrophysics (course requirements and thesis research). From my personal experience, I cannot stress enough the advantage that it is for my current research to have a degree in physics because it gave me the mathematical background and fundamental knowledge to tackle any problem.