  Powerful flares from blazars with short ($\sim$ min) variability timescales
are challenging for current models of blazar emission. Here, we present a
physically motivated ab initio model for blazar flares based on the results of
recent particle-in-cell (PIC) simulations of relativistic magnetic
reconnection. PIC simulations demonstrate that quasi-spherical plasmoids filled
with high-energy particles and magnetic fields are a self-consistent by-product
of the reconnection process. By coupling our PIC-based results (i.e., plasmoid
growth, acceleration profile, particle and magnetic content) with a kinetic
equation for the evolution of the electron distribution function we demonstrate
that relativistic reconnection in blazar jets can produce powerful flares whose
temporal and spectral properties are consistent with the observations. In
particular, our model predicts correlated synchrotron and synchrotron
self-Compton flares of duration of several hours--days powered by the largest
and slowest moving plasmoids that form in the reconnection layer. Smaller and
faster plasmoids produce flares of sub-hour duration with higher peak
luminosities than those powered by the largest plasmoids. Yet, the observed
fluence in both types of flares is similar. Multiple flares with a range of
flux-doubling timescales (minutes to several hours) observed over a longer
period of flaring activity (days or longer) may be used as a probe of the
reconnection layer's orientation and the jet's magnetization. Our model shows
that blazar flares are naturally expected as a result of magnetic reconnection
in a magnetically-dominated jet.
