  It is commonly believed that blazar jets are relativistic magnetized plasma
outflows from supermassive black holes. One key question is how the jets
dissipate magnetic energy to accelerate particles and drive powerful
multi-wavelength flares. Relativistic magnetic reconnection has been proposed
as the primary plasma physical process in the blazar emission region. Recent
numerical simulations have shown strong acceleration of nonthermal particles
that may lead to multi-wavelength flares. Nevertheless, previous works have not
directly evaluated $\gamma$-ray signatures from first-principle simulations. In
this paper, we employ combined particle-in-cell and polarized radiation
transfer simulations to study multi-wavelength radiation and optical
polarization signatures under the leptonic scenario from relativistic magnetic
reconnection. We find harder-when-brighter trends in optical and {\it
Fermi-LAT} $\gamma$-ray bands as well as closely correlated optical and
$\gamma$-ray flares. The optical polarization angle swings are also accompanied
by $\gamma$-ray flares with trivial time delays. Intriguingly, we find highly
variable synchrotron self Compton signatures due to inhomogeneous particle
distributions during plasmoid mergers. This feature may result in fast
$\gamma$-ray flares or orphan $\gamma$-ray flares under the leptonic scenario,
complementary to the frequently considered mini-jet scenario. It may also infer
neutrino emission with low secondary synchrotron flux under the hadronic
scenario, if plasmoid mergers can accelerate protons to very high energy.
