  Beams of ultra-relativistic electrons in blazar jets develop pair cascades
interacting with ambient soft photons. Employing coupled kinetic equations with
escape terms, we model the unsaturated pair cascade spectrum. We assume that
the gamma rays predominantly scatter off recombination-line photons from clouds
photoionised by the irradiation from the accretion disk and the jet. The
cascade spectrum is rather insensitive to the injection of hard electron
spectra associated with the short-time variability of blazars. Adopting
physical parameters representative of Markarian 501 and 3C 279, respectively,
we numerically obtain spectral energy distributions showing distinct features
imprinted by the recombination-line photons. The hints for a peculiar feature
at ~3 TeV in the spectrum of Markarian 501, detected with the MAGIC telescopes
during a strong X-ray flux activity in 2014 July, can be explained in this
scenario as a result of the up-scattering of line photons by beam electrons and
the low pair-creation optical depth. Inspecting a high-fidelity Fermi-LAT
spectrum of 3C 279 from January 2018 reveals troughs in the spectrum that
coincide with the threshold energies for gamma rays producing pairs in
collisions with recombination-line photons, and the absence of exponential
attenuation. Our finding implies that the gamma rays in 3C 279 escape from the
edge of the broad emission line region.
