  We consider the observational properties of a static black hole space-time
immersed in a dark matter envelope. We thus investigate how the modifications
to geometry, induced by the presence of dark matter affect the luminosity of
the black hole's accretion disk. We show that the same disk's luminosity
produced by a black hole in vacuum may be produced by a smaller black hole if
surrounded by dark matter under certain conditions. In particular, we
demonstrate that the luminosity of the disk is markedly altered by dark
matter's presence, suggesting that mass estimation of distant super-massive
black holes may be changed if they are immersed in dark matter. We argue that a
similar effect holds in more realistic scenarios and we discuss about the
refractive index related to dark matter lensing. Hence we show how this may
help explain the observed luminosity of super-massive black holes in the early
universe.
