Many quasi-simultaneous optical observations of 37 blazars were obtained at the Perugia University Observatory over the last ten years. In this paper we analyze the dereddened spectral flux distribution in the optical frequency range, and we compare our results with the overall spectral energy distribution (SED). The implications for existing models of the objects are briefly discussed. In particular, we show how the LBLs have a spectral slope α ≃ 1.5, as expected from Synchrotron Self-Compton models, while the optical emission of HBLs and FSRQs is probably contaminated by other components. Moreover, we show evidence of how the spectral index vs. flux plot exhibits a characteristic loop-like pattern that is probably due to changes in the injection of accelerated particles.
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