Blackhole Finding Techniques I know of two methods for finding exoplanets: the transit method and the radial velocity method. These two methods work as follows:


Transit: we observe stars and watch for when a planet obstructs the light from the star.
Radial Velocity: detecting the planet by observing the motion of the star and using Kepler's law.


Are these methods also used for discovering black-holes? What are the pros and cons of these methods, I think that the con of transit is that it's difficult to see the black hole. A pro is that it doesn't matter how many stars there are around the black hole.

With the Radial Velocity method, a pro is that it's easy to detect but requires exactly one companion star (I used this source).
We detect black holes mostly by their effects on nearby matter. The "radial velocity" method is important after x-ray observations suggest a likely black-hole.

Matter in orbit around a black hole can form an accretion disc. This disc will become very hot, and glow brightly in X-ray radiation. If we see X-ray radiation coming from a star we look at the radial velocity of the star, and use this to infer the mass of the orbiting object. Since a neutron star cannot exist at more than 3 solar masses, if the object is more than that it must be a black hole.

Compared to planets, black holes are rare. Only about 20 candidates are known in the Milky way.