The idea of a body so massive that even light could not escape was first proposed in the late 18th century by English astronomer and clergyman John Michell and independently by French scientist Pierre-Simon Laplace. Both scholars proposed very large stars in contrast to the modern concept of an extremely dense object.

Michell’s idea, in a short part of a letter published in 1784,[6] calculated that a star with the same density but 500 times the radius of the sun would not let any emitted light escape; the surface escape velocity would exceed the speed of light.

what’s funny is that he even got the radius (schwarzschild-radius) right. just plug in c for the escape velocity and you get the schwarzschild-radius.