  Imaging the immediate vicinity of supermassive black holes (SMBH) and
extracting a BH-spin signature is one of the grand challenges in astrophysics.
M87 is known as one of the best targets for imaging the BH shadow and it can be
partially thick against synchrotron self-absorption (SSA), particularly in a
flaring state with high mass accretion rate. However, little is known about
influences of the SSA-thick region on BH shadow images. Here we investigate BH
shadow images of M87 at 230 GHz properly taking into account the SSA-thick
region. When the BH has a high spin value, the corresponding BH shadow image
shows the positional offset between the center of the photon ring and that of
the SSA-thick ring at the innermost stable circular orbit (ISCO) due to the
frame-dragging effect in the Kerr spacetime. As a result, we find that a
dark-crescent structure is generally produced between the photon ring and the
SSA-thick ISCO ring in the BH shadow image. The scale size of the dark-crescent
increase with BH spin: its width reaches up to $\sim 2$ gravitational radius
when the BH spin is 99.8% of its maximum value. The dark crescent is regarded
as a new signature of a highly spinning BH. This feature is expected to appear
in flaring states with relatively high mass accretion rate rather than the
quiescent states. We have simulated the image reconstruction of our theoretical
image by assuming the current and future Event Horizon Telescope (EHT) array,
and have found that the future EHT including space-very long baseline
interferometry in 2020s can detect the dark crescent.
