  In general relativistic magneto-hydrodynamic (GRMHD) simulations, accreted
magnetic flux on the black hole horizon episodically decays, during which
magnetic reconnection heats up the plasma near the horizon, potentially
powering high-energy flares like those observed in M87* and Sgr A*. We study
the mm observational counterparts of such flaring episodes. The change in 230
GHz flux during the expected high energy flares depends primarily on the
efficiency of accelerating $\gamma \gtrsim 100$ ($T_e \gtrsim 10^{11}$ K)
electrons. For models in which the electrons are heated to $T_e \sim 10^{11}$ K
during flares, the hot plasma produced by reconnection significantly enhances
230 GHz emission and increases the size of the 230 GHz image. By contrast, for
models in which the electrons are heated to higher temperatures (which we argue
are better motivated), the reconnection-heated plasma is too hot to produce
significant 230 GHz synchrotron emission, and the 230 GHz flux decreases during
high energy flares. We do not find a significant change in the mm polarization
during flares as long as the emission is Faraday thin. We also present
expectations for the ring-shaped image as observed by the Event Horizon
Telescope during flares, as well as multi-wavelength synchrotron spectra. Our
results highlight several limitations of standard post-processing prescriptions
for the electron temperature in GRMHD simulations. We also discuss the
implications of our results for current and future observations of flares in
Sgr A*, M87*, and related systems. Appendices contain detailed convergence
studies with respect to resolution and plasma magnetization.
