  The mid-infrared (MIR; $\lambda\simeq3 - 10\mu$m) bands offer a unique window
into understanding accretion and its interplay with jet formation in Galactic
black hole X-ray binaries (BHXRBs). Although extremely difficult to observe
from the ground, the NEOWISE time domain survey offers an excellent data set to
study MIR variability when combined with contemporaneous X-ray data from the
MAXI all-sky survey over a $\approx15$ yr baseline. Using a new forced
photometry pipeline for NEOWISE data, we present the first systematic study of
BHXRB MIR variability in outburst. Analyzing a sample of 16 sources detected in
NEOWISE, we show variability trends in the X-ray hardness and MIR spectral
index wherein i) the MIR bands are typically dominated by jet emission during
the hard states, constraining the electron power spectrum index to $p \approx
1-4$ in the optically thin regime and indicating emitting regions of a few tens
of gravitational radii when evolving towards a flat spectrum, ii) the MIR
luminosity ($L_{IR}$) scales as $L_{IR}\propto L_X^{0.82\pm0.12}$ with the
$2-10$ keV X-ray luminosity ($L_X$) in the hard state, consistent with its
origin in a jet, and iii) the thermal disk emission dominates the soft state as
the jet switches off and dramatically suppresses ($\gtrsim 10\times$) the MIR
emission into a inverted spectrum ($\alpha\approx -1$, where
$F_\nu\propto\nu^{-\alpha}$). We highlight a population of `mini' BHXRB
outbursts detected in NEOWISE (including two previously unreported episodes in
MAXI J1828-249) but missed in MAXI due to their faint fluxes or source
confusion, exhibiting MIR spectral indices suggestive of thermal emission from
a large outer disk. We highlight that upcoming IR surveys and the Rubin
observatory will be powerful discovery engines for the distinctively large
amplitude and long-lived outbursts of BHXRBs, as an independent discovery route
to X-ray monitors.
