  We present the discovery of a new optical/X-ray source likely associated with
the Fermi $\gamma$-ray source 4FGL J1408.6-2917. Its high-amplitude periodic
optical variability, large spectroscopic radial velocity semi-amplitude,
evidence for optical emission lines and flaring, and X-ray properties together
imply the source is probably a new black widow millisecond pulsar binary. We
compile the properties of the 41 confirmed and suspected field black widows,
finding a median secondary mass of $0.027\pm0.003\,M_{\odot}$. Considered
jointly with the more massive redback millisecond pulsar binaries, we find that
the "spider" companion mass distribution remains strongly bimodal, with
essentially zero systems having companion masses between
$\sim0.07-0.1\,M_{\odot}$. X-ray emission from black widows is typically softer
and less luminous than in redbacks, consistent with less efficient particle
acceleration in the intrabinary shock in black widows, excepting a few systems
that appear to have more efficient "redback-like" shocks. Together black widows
and redbacks dominate the census of the fastest-spinning field millisecond
pulsars in binaries with known companion types, making up $\gtrsim$80% of
systems with $P_{\rm{spin}}<2\,\rm{ms}$. Similar to redbacks, the neutron star
masses in black widows appear on average significantly larger than the
canonical $1.4\,M_{\odot}$, and many of the highest-mass neutron stars claimed
to date are black widows with $M_{\rm{NS}}\gtrsim2.1\,M_{\odot}$. Both of these
observations are consistent with an evolutionary picture where spider
millisecond pulsars emerge from short orbital period progenitors that had a
lengthy period of mass transfer initiated while the companion was on the main
sequence, leading to fast spins and high masses.
