  Future space observatories will likely have segmented primaries, causing
diffraction effects that reduce coronagraph performance. Reflective binary
pupil apodizer masks can mitigate these, with the metamaterial black silicon
(BSi) showing promise as a strong absorber. To bring contrast ratios to the
$10^-{10}$ level as needed to observe Earth-like exoplanets, feature sizes on
these BSi masks will need to be less than $5$ microns when paired with MEMS
(micro-electromechanical systems) deformable mirrors. As scalar diffraction
cannot reliably model this feature size, we developed a Finite-Difference
Time-Domain (FDTD) model of BSi masks using Meep software. We characterize the
FDTD-derived polarization-dependent bidirectional reflectance distribution
function of BSi and discuss the model's shortcomings.
