Can we really photograph a black hole? Are they not entirely dark, since no light can escape them?

The first image of a black hole is not a classical photograph. It is a radiolight image the result of complex observational and computational interpretation (deconvolution). Further, it is not of the black hole itself, but of the "shadow"—the closest we can come to imaging a completely dark object that consumes all light and matter. The black hole boundary—the event horizon for which the EHT is named—casts this shadow. General Relativity says the superheated material around the black hole will glow and illuminate the strongly warped region of spacetime, making it visible to interferometer observation and measurement.

This animation and this article may help in better understanding how the image of a black hole is formed.