Once you know speed of light is the maximum possible speed, you can try to guess what happens when you have enough mass to make escape velocity higher than the speed of light (it is not possible, therefore you can't escape).
If you don't limit yourself to entirely ballistic trajectories you can escape without ever reaching that point's escape velocity.
To explain why nothing, not even things on non-ballistic trajectories, cannot get out of black holes you've got to turn to weird headache inducing general relativity stuff.
It's much easier to think of the gradual warping of spacetime around a black hole so that more and more directions are down. East? No, that's really east and down. Left? Nope, you thought you were going left but it is left and down. Less and less "up" is available to you until you are on a knife edge where you can make an orbit (assuming a non-rotating black hole, no angular momentum) around the event horizon with a photon. Beyond that, everything is some flavor of down.
That when mass becomes high enough that light can't escape nothing else can escape either doesn't pop out of the math until you have general relativity.
Unless the rocket collapses into a black hole under the sheer mass of all that fuel.
One would certainly not expect elementary particles to function like rockets.
But yeah, nobody could come with a description of a black hole anything like the modern one before 1900. Anything older is certain to be very different.
Imagine accelerating an electron in some sort of magnetic field, in series. In a Newtonian system any speed is achievable.
GR forbids this though.
Does it though?
Relativity makes black holes much more "final" than this original concept, but he was definitely thinking in the right direction.
The wave theory of light came quite a bit later, in Maxwell's time.
It's not a question of enough mass. It's a question of being compact enough. There is no minimum mass for a black hole, but the smaller the mass, the smaller the surface area of the horizon (the 2-sphere that the mass has to collapse inside to form the hole).
But he didn't postulate this at all. He simply postulated (as per Newton) that light is made of particles with mass, which could thus be affected by gravity. Since they are moving at a finite speed, the light particles would be slowed down by gravity, and eventually completely stopped.
He further proposed, based on this model, that the mass of an object could potentially be determined by determining how much slower the particles were moving.
This doesn't work. Light doesn't really get slowed down by gravity, and you definitely don't have any slow light that was created from stars trillions of miles away.
It's a cool idea, but doesn't work with how light actually propagates.