The present study employs the FTIR spectroscopy methods, such as polarized transmittance measurements and the reflectance anisotropy spectroscopy technique, to characterize the optical properties of black phosphorus - a layered semiconductor with a narrow band gap. Our results reveal a notable crystal absorption anisotropy within the 0.26-0.42 eV range with strong linear dichroism, wherein a polarization-dependent feature is observed in the reflectance anisotropy spectra with a maximum near 0.33 eV. This feature is believed to be related to a direct interband transition E0, which is permitted for linearly polarized incident radiation along the AC crystal direction and forbidden for the ZZ direction.