What boundary around a black hole marks the point beyond which light cannot escape?

The event horizon is the boundary around a black hole beyond which light cannot escape.

In general relativity, the event horizon is not a solid surface. It is a boundary in spacetime: once an object or light signal crosses inward, every future-directed path leads deeper into the black hole, so no information can return to the outside. For a non-rotating black hole, the horizon lies at the Schwarzschild radius.

The event horizon is often confused with nearby features. An accretion disk is hot material orbiting outside the black hole, while the photon sphere is a region where light can follow unstable circular paths. A rotating black hole also has an ergosphere outside its event horizon, where spacetime is dragged around by the black hole's spin; light and matter can still escape from the ergosphere.

A distant observer sees an infalling object appear to slow and fade because of gravitational time dilation and redshift. That visual effect does not mean the object physically stops at the horizon. An observer falling with it would cross the horizon in finite proper time, although the outcome depends on the black hole's size and tidal forces.