  Dark matter sectors with hidden interactions have been of much interest in
recent years. These frameworks include models of millicharged particles as well
as dark sector bound states, whose constituents have electromagnetic gauge
interactions. These exotic, charged states could constitute a part of the total
dark matter density. In this work, we explore in some detail the various
effects, on the photon sphere and shadow of spherically symmetric black holes,
due to dark matter plasmas furnished by such sectors. Estimating physically
viable parameter spaces for the particle physics models and taking
semi-realistic astrophysical scenarios that are amenable to theoretical
analyses, we point out various modifications and characteristics that may be
present. Many of these effects are unique and very distinct from analogous
situations with conventional baryonic plasmas, or neutral perfect fluid dark
matter surrounding black holes. While in many physically viable regions of the
parameter space the effects on the near-horizon regions and black hole shadows
are small, in many parts of the low particle mass regions the effects are
significant, and potentially measurable by current and future telescopes. Such
deviations, for instance, include characteristic changes in the photon sphere
and black hole shadow radii, unique thresholds for the dark matter plasma
dispersion where the photon sphere or black hole shadow vanishes, and where the
dark matter plasma becomes opaque to electromagnetic waves. Alternatively, we
point out that a non-observation of such deviations and characteristics, in
future, could put constraints on interesting regions of the particle physics
parameter space.
