As the accreting matter approaches the event horizon, it gets accelerated to near light speed by the black hole’s gravitational pull.
If the inflow rate of matter is high and collisions occur within the flow, then the matter is heated and radiates.
This radiation is what you see in the illustrations. Lots of black holes without this radiation must exist that astronomers can’t see.
The radiation is most intense within the innermost regions, with about half the radiation originating from within five to 15 times the event horizon’s radius, depending on how the black hole is spinning.
The feeding black hole remains completely black while lying at the center of the brightest persistent emission in the universe.
University of Cambridge, England