The black region is the shadow: light from there would have to come out of the event horizon.

The arc over the top is the far side of the accretion disk, its light bent over the hole. The ring underneath is the same disk seen from below.

The left side is brighter and bluer because that gas orbits towards you at about half the speed of light. The receding right side is dimmer and redder.

The thin bright line hugging the shadow is the photon ring: light that circled the hole before escaping.