  We present an analysis of the ultraviolet (UV) continuum slopes ($\beta$) for
a sample of $172$ galaxy candidates at $8 < z_{\mathrm{phot}} < 16$ selected
from a combination of JWST NIRCam imaging and COSMOS/UltraVISTA ground-based
near-infrared imaging. Focusing primarily on a new sample of $121$ galaxies at
$\langle z \rangle \simeq 11$ selected from $\simeq 320$ arcmin$^2$ of public
JWST imaging data across $15$ independent data sets, we investigate the
evolution of $\beta$ in the galaxy population at $z \geq 9$. We find a
significant trend between $\beta$ and redshift, with the inverse-variance
weighted mean UV slope evolving from $\langle \beta \rangle = -2.17 \pm 0.06$
at $z = 9.5$ to $\langle \beta \rangle = -2.59 \pm 0.06$ at $z = 11.5$. Based
on a comparison with stellar population models including nebular continuum
emission, we find that at $z>10.5$ the average UV continuum slope is consistent
with the intrinsic blue limit of dust-free stellar populations
$(\beta_{\mathrm{int}} \simeq -2.6)$. These results suggest that the moderately
dust-reddened galaxy population at $z < 10$ was essentially unattenuated at $z
\simeq 11$. The extremely blue galaxies being uncovered at $z>10$ place
important constraints on dust attenuation in galaxies in the early Universe,
and imply that the already observed galaxy population is likely supplying an
ionising photon budget capable of maintaining ionised IGM fractions of $\gtrsim
5$ per cent at $z\simeq11$.
