  The launch of NuSTAR and the increasing number of binary black hole (BBH)
mergers detected through gravitational wave (GW) observations have
exponentially advanced our understanding of black holes. Despite the simplicity
owed to being fully described by their mass and angular momentum, black holes
have remained mysterious laboratories that probe the most extreme environments
in the Universe. While significant progress has been made in the recent decade,
the distribution of spin in black holes has not yet been understood. In this
work, we provide a systematic analysis of all known black holes in X-ray binary
systems (XB) that have previously been observed by NuSTAR, but have not yet had
a spin measurement using the "relativistic reflection" method obtained from
that data. By looking at all the available archival NuSTAR data of these
sources, we measure ten new black hole spins: IGR J17454-2919 --
$a=0.97^{+0.03}_{-0.17}$; GRS 1758-258 -- $a=0.991^{+0.007}_{-0.019}$; MAXI
J1727-203 -- $a=0.986^{+0.012}_{-0.159}$; MAXI J0637-430 -- $a=0.97\pm0.02$;
Swift J1753.5-0127 -- $a=0.997^{+0.001}_{-0.003}$; V4641 Sgr --
$a=0.86^{+0.04}_{-0.06}$; 4U 1543-47 -- $a=0.98^{+0.01}_{-0.02}$; 4U 1957+11 --
$a=0.95^{+0.02}_{-0.04}$; H 1743-322 -- $a=0.98^{+0.01}_{-0.02}$; MAXI
J1820+070 -- $a=0.988^{+0.006}_{-0.028}$ (all uncertainties are at the
$1\sigma$ confidence level). We discuss the implications of our measurements on
the entire distribution of stellar mass black hole spins in XB, and we compare
that with the spin distribution in BBH, finding that the two distributions are
clearly in disagreement. Additionally, we discuss the implications of this work
on our understanding of how the "relativistic reflection" spin measurement
technique works, and discuss possible sources of systematic uncertainty that
can bias our measurements.
