From a relativistic point of view, the concept of a black hole was theorized from the physicist Karl Schwarzschild in 1916, only a year after publication of the general theory of relativity. In general relativity the gravitational field is described as deformation of space-time caused by a very massive object and the speed of light is a constant limit.
Exploring some solutions to the equations of the theory of general relativity, Schwarzschild calculated that a body, hypothetically endowed with very high density, would produce in its vicinity a deformation such that the light away from it it would tend to undergo an infinite gravitational red shift. This means that gravity is so strong that the vision of light is impossible.
It took Albert Einstein eight years, from 1907 to 1915, to complete his general theory of relativity. His work in these years was driven by ingenious thought experiments, forays into new mathematics, tedious calculations, painful missteps, dogged persistence and moments of great exhilaration.
Einstein's Discovery of the General Theory of Relativity