The GeV-range spectra of blazars are shaped not only by non-thermal
emission processes internal to the relativistic jet but also by external
pair-production absorption on the thermal emission of the accretion disc
and the broad-line region (BLR). For the first time, we compute here the
pair-production opacities in the GeV range produced by a realistic BLR
accounting for the radial stratification and radiation anisotropy. Using
photoionization modelling with the CLOUDY code, we calculate a series of
BLR models of different sizes, geometries, cloud densities, column
densities and metallicities. The strongest emission features in the
model BLR are Ly α and He II Ly α. Contribution of
recombination continua is smaller, especially for hydrogen, because Ly
continuum is efficiently trapped inside the large optical depth BLR
clouds and converted to Lyman emission lines and higher order
recombination continua. The largest effects on the gamma-ray opacity are
produced by the BLR geometry and localization of the gamma-ray source.
We show that when the gamma-ray source moves further from the central
source, all the absorption details move to higher energies and the
overall level of absorption drops because of decreasing incidence angles
between the gamma-rays and BLR photons. The observed positions of the
spectral breaks can be used to measure the geometry and the location of
the gamma-ray emitting region relative to the BLR. Strong dependence on
geometry means that the soft photons dominating the pair-production
opacity may be actually produced by a different population of BLR clouds
than the bulk of the observed broad line emission.