Is it not the density and gravitational pull that keeps light from escaping that makes them black?
The speed of light is the speed of causality first. The area beyond the event horizon is black because we can have no causal connection to the region inside. To outside observers, nothing ever actually happens inside and if we watched someone fall in they would freeze at the event horizon because their gravitational accceleration would equal the speed of light. Look up the photon clock thought experiment, time appears to stop to outside observers for something traveling at the speed of light.
The density and gravitational pull is what sufficiently warp spacetime to erase the region from our causal connection. It can be calculated for any size body and is called the Schwartzshield radius. Generally takes the mass of a large star collapsing to form a black hole though.