On November 27th, 1783, John Michell first proposed the idea that there were such things as [black holes] which he called “dark stars” two hundred years before anyone else. He correctly deduced they would be created by extreme gravity.
Big Al didn’t do much with black hole theory - Schwarzschild, Eddington and Oppenheimer (and later Kerr, Newman and Hawking of course) were more important in that area (though they all worked with Einstein’s equations of course). Einstein himself didn’t believe the black holes were physically possible.
Michell based his speculation on the corpuscular theory of light, i.e. that light consists of particles that are attracted by gravity. He thought that when the escape velocity was greater than the speed of light, particles of light would fall back to the star. This is incorrect.
In the 1860s Maxwell showed that light is an electromagnetic wave. Later the wave-particle duality was discovered.
There’s a superficial similarity between the ideas of Michell and Einstein, but in fact they are very different. General Relativity predicts a bending of spacetime in the vicinity of mass, which is the reason the trajectories of photons are bent, even though photons are massless.