A black hole is an object with such a strong gravitational field that even light cannot escape from its surface. A black hole may be formed when a massive object (very big object) undergoes uncontrolled contraction (collapse) because of the inward pull of its own gravity.
It is black because it has a strong gravitational field in which light cannot escape from its surface or even any light pass on its surface. A black hole in galaxy comes into the view when a star completes its entire life and dies that means when the star uses all its energy and lastly converts into a black hole with a strong gravitational pull.
The neutron stars shrink so much and become so dense that the resulting black holes do not allow any light to escape from its surface. This is because the black holes have a tremendous gravitational force. Since even light cannot escape from its surface, therefore, black holes are invisible so that they cannot be seen with naked eyes. The presence of a black hole can be felt only by the effect of its gravitational field on its neighboring objects in the sky.