The recent astonishing achievement of obtaining an actual image of a blackhole, resulted from a massive collaborative work using 8 different telescopes and the equally on par efforts in developing the imaging technique and data analysis algorithms, which were primarily driven by works of Katie Bouman, needed to synchronize and correct all gathered data and transform into a unified image.
As far as I understand, the telescopes were able to capture the EM radiation emitted from the friction of matter orbiting around (towards) the blackhole, which given its pull strength leads to very strong friction forces that result in highly bright radiation.
Admittedly, it is very difficult to grasp the work for someone not involved in the field, but it would be incredibly valuable if, briefly and at a rough conceptual level, the repercussions of these new findings can be put into perspective compared to what was known before. More precisely,
in what new ways does the image (and the fact that it could be captured) validate our understanding of GR? In contrast to the previous corroborations of blackholes, namely, the gravitational effect of them on the orbit of nearby stars and planets or the gravitational waves produced upon the collapse of two blackholes.
At a more technical level, the captured EM radiation travelling for such a long distance ($\approx 54$ million light years), I imagine it must have been strongly redshifted considering the Doppler effect, expansion of the universe and the gravitational pull of the blackhole and of all intermediate stars on the radiation, in spite of that it was still possible to distinguish the radiation spectrum and recognize its source without confusing it with any other source or the radio-wave background. Does the redshifting bear no additional challenge in identifying the source of a radiation such as in this case?
This is merely an attempt to get more input in order to understand at a basic level some of the main repercussions of having successfully imaged a blackhole, as this is all very exciting news to learn more about.