We demonstrate that the spectral differences between Flat Spectrum Radio Quasars (FSRQ) with steep gamma ray spectra (MeV-blazars) and FSRQ with flat gamma ray spectra (GeV-blazars) can be explained by assuming that in the MeV-blazars, the production of gamma-rays is dominated by Comptonization of infrared radiation of hot dust, whereas in the GeV-blazars--by Comptonization of broad emission lines. Additional ingredient, required to reach satisfactory unification, is an assumption that the radiating electrons are accelerated via a two step process, in the lower energy range--following instabilities driven by shock-reflected ions, and in the higher energy range--via resonant scatterings off Alfven waves. Our model predicts that on average, the MeV-blazars should vary on longer time scales than GeV-blazars.