[yk]

This is as far as I understand the first picture of a black hole shadow, that is the dark disc that indicates that there is actually light missing. Before that, there were astrophysical observations that strongly indicated that there is a black hole, for example QSOs, that is a bright combination of accretion disc and jet, that is powered by a black hole, or one paper I really liked tried to argue that there has to be a black hole in Cygni X1 because otherwise we would see the missing matter crashing into something. (Cygni X1 is a binary of a star and something very bright and the star is loosing mass.)