  Accreting black holes tend to display a characteristic dark central region
called the black-hole shadow, which depends only on spacetime/observer geometry
and which conveys information about the black hole's mass and spin. Conversely,
the observed central brightness depression, or image shadow, additionally
depends on the morphology of the emission region. In this paper, we investigate
the astrophysical requirements for observing a meaningful black-hole shadow in
GRMHD-based models of accreting black holes. In particular, we identify two
processes by which the image shadow can differ from the black-hole shadow:
evacuation of the innermost region of the accretion flow, which can render the
image shadow larger than the black-hole shadow, and obscuration of the
black-hole shadow by optically thick regions of the accretion flow, which can
render the image shadow smaller than the black-hole shadow, or eliminate it
altogether. We investigate in which models the image shadows of our models
match their corresponding black-hole shadows, and in which models the two
deviate from each other. We find that, given a compact and optically thin
emission region, our models allow for measurement of the black-hole shadow size
to an accuracy of 5%. We show that these conditions are generally met for all
MAD simulations we considered, as well as some of the SANE simulations.
