  We consider a simple spherical model consisting of a Schwarzschild black hole
of mass $M$ and a dark matter of mass $\Delta M$ around it. The general formula
for the radius of black-hole shadow has been derived in this case. It is shown
that the change of the shadow is not negligible, once the effective radius of
the dark matter halo is of order $\sim \sqrt{3 M \Delta M}$. For this to
happen, for example, for the galactic black hole, the dark matter must be
concentrated near the black hole. For small deviations from the Schwarzschild
limit, the dominant contribution into the size of a shadow is due to the dark
matter under the photon sphere, but at larger deviations, the matter outside
the photon sphere cannot be ignored.
