Lets think about Sound, After sudden frequency we can not hear it. We have hearing range ( 20 to 20,000 Hz ) within which we can hear. Likewise When Source light become more and more strong it becomes more difficult for our eye or cameras to process it. So it looks dark.
You may think that powerful light must damage our eye. But I am not talking about that. What I want to say is that The light from where it emitting (the source) must overcome something or some properties that we or our camera can not process it.
You may take an example- TV remote control Light
( It is my thought only )
Actually I am pointing on so called blackhole. blackhole emits light but some light property may be destroyed or supper increased that it become dark to us.