  Using the density functional theory of electronic structure, we compute the
anisotropic dielectric response of bulk black phosphorus subject to strain.
Employing the obtained permittivity tensor, we solve Maxwell's equations and
study the electromagnetic response of a layered structure comprising a film of
black phosphorus stacked on a metallic substrate. Our results reveal that a
small compressive or tensile strain, $\sim 4\%$, exerted either perpendicular
or in the plane to the black phosphorus growth direction, efficiently controls
the epsilon-near-zero response, and allows a perfect absorption tuning from
low-angle of the incident beam $\theta=0^\circ$ to high values $\theta\approx
90^\circ$ while switching the energy flow direction. Incorporating a spatially
inhomogeneous strain model, we also find that for certain thicknesses of the
black phosphorus, near-perfect absorption can be achieved through controlled
variations of the in-plane strain. These findings can serve as guidelines for
designing largely tunable perfect electromagnetic wave absorber devices.
