Blazars radiate from relativistic plasma jets with bulk Lorentz factors \Gamma \sim 10 , closely aligned along our line of sight .
In a number of blazars of the Flat Spectrum Radio Quasar type such as 3C 454.3 and 3C 279 gamma-ray flares have recently been detected with very high luminosity and little or no counterparts in the optical and soft X-ray bands .
They challenge the current one-zone leptonic models of emissions from within the broad line region .
The latter envisage the optical/X-ray emissions to be produced as synchrotron radiation by the same population of highly relativistic electrons in the jet that would also yield the gamma rays by inverse Compton up-scattering of surrounding soft photons .
To meet the challenge we present here a model based on primary synchrotron photons emitted in the broad line region by a plasmoid moving out with the jet and scattered back toward the incoming plasmoid by an outer plasma clump acting as a mirror .
We consider both a scenario based on a static mirror located outside the BLR , and an alternative provided by a moving mirror geometry .
We show that mirroring phenomena can locally enhance the density and anisotropy with associated relativistic boosting of soft photons within the jet , so as to trigger bright inverse Compton gamma-ray transients from nearly steady optical/X-ray synchrotron emissions .
In this picture we interpret the peculiarly asymmetric lightcurves of the recently detected gamma-ray flares from 3C 279 .
Our scenario provides a promising start to understand the widening class of bright and transient gamma-ray activities in blazars .