  Shadows of black holes surrounded by an optically thin emitting medium have
been extensively discussed in the literature. The Hioki-Maeda algorithm is a
simple recipe to characterize the shape of these shadows and determine the
parameters of the system. Here we extend their idea to the case of a dressed
black hole, namely a black hole surrounded by a geometrically thin and
optically thick accretion disk. While the boundary of the shadow of black holes
surrounded by an optically thin emitting medium corresponds to the apparent
photon capture sphere, that of dressed black holes corresponds to the apparent
image of the innermost stable circular orbit. Even in this case, we can
characterize the shape of the shadow and infer the black hole spin and viewing
angle. The shape and the size of the shadow of a dressed black hole are
strongly affected by the black hole spin and inclination angle. Despite that,
it seems that we cannot extract any additional information from it. Here we
study the possibility of testing the Kerr metric. Even with the full knowledge
of the boundary of the shadow, those of Kerr and non-Kerr black holes are very
similar and it is eventually very difficult to distinguish the two cases.
