Blazars are strongly variable sources that occasionally show spectacular flares visible in various energy bands. These flares are often, but not always, correlated. In a number of cases the peaks of optical flares are found to be somewhat delayed with respect to the gamma-ray peaks. One notable example of such a delay was found in 3C 279 by Hayashida et al. and interpreted as a result of steeper drop with distance of the energy density of external radiation field than of the magnetic energy density. In this paper we demonstrate that in general, depending on the respective energy density profile along the jet, such lags can have both signs and that they can take place for any ratio of these energy densities. We study the dependence of such lags on the ratio of these energy densities at a distance of a maximal energy dissipation in a jet, on their gradients, as well as on the time profile of the relativistic electron injection within the moving source. We show how prominent such lags can be, and what are their expected time scales. We suggest that studies of such lags can provide a powerful tool to resolve the structure of relativistic jets and their radiative environment. As an example we model the lag observed in 3C 279, showing that in this object the flare is produced at a distance of a few parsecs from the central black hole, consistent with our previous inferences based on the spectra and optical polarization properties.
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