  Crystalline black nitrogen (BN) is an allotrope of nitrogen with the black
phosphorus (BP) structure recently synthesized at high pressure by two
independent research groups [Ji et al., Sci. Adv. 6, eaba9206 (2020); Laniel et
al., Phys. Rev. Lett. 124, 216001 (2020)]. Here, we present a systematic study
of the electronic and optical properties of BN focusing on its comparison with
BP. To this end, we use the state-of-the-art quasiparticle self-consistent $GW$
approach with vertex corrections in both the electronic and optical channels.
Despite many similarities, the properties of BN are found to be considerably
different. Unlike BP, BN exhibits a larger optical gap (2.5 vs 0.26 eV), making
BN transparent in the visible spectral region with a highly anisotropic optical
response. This difference can be primarily attributed to a considerably reduced
dielectric screening in BN, leading to enhancement of the effective Coulomb
interaction. Despite relatively strong Coulomb interaction, exciton formation
is largely suppressed in both materials. Our analysis of the elastic properties
shows exceptionally high stiffness of BN, comparable to that of diamond.
