  Blazars may accelerate protons and/or nuclei as well as electrons. The
hadronic component of accelerated particles in blazars may constitute the bulk
of their high-energy budget; nevertheless, this component is elusive due to a
high value of the energy threshold of proton interaction with photon fields
inside the source. However, broad line regions (BLRs) of some flat spectrum
radio quasars (FSRQs) may contain a sufficient amount of matter to render
primary protons "visible" in $\gamma$ rays via hadronuclear interactions. In
the present paper we study the persistent $\gamma$-ray emission of the FSRQ PKS
1510-089 in its low state utilizing the publicly-available Fermi-LAT data, as
well as using the spectrum measured with the MAGIC imaging atmospheric
Cherenkov telescopes. We find an indication for an excess of $\gamma$ rays at
the energy range $\gtrsim 20$ GeV with respect to a simple baseline
log-parabolic intrinsic spectral model. This excess could be explained in a
scenario invoking hadronuclear interactions of primary protons on the BLR
material with the subsequent development of electromagnetic cascades in photon
fields. We present a Monte Carlo calculation of the spectrum of this cascade
component, taking as input the BLR photon field spectrum calculated with the
Cloudy code. To our knowledge, this is the first calculation of electromagnetic
cascade spectrum inside a blazar based on a direct calculation of the photon
field spectrum with a spectral synthesis code.
