  The observation of a radioactively-powered kilonova associated with the first
binary neutron star (BNS) merger detected in gravitational waves proved that
these events are ideal sites for the production of heavy r-process elements.
However, the physical origin of the ejected material responsible for the early
("blue") and late ("red") components of this kilonova is still debated. Here,
we investigate the possibility that the early/blue kilonova originated from the
magnetically driven baryon wind launched after merger by the metastable neutron
star remnant. Exploiting a magnetized BNS merger simulation with over 250 ms of
post-merger evolution, we can follow for the first time the full mass ejection
process up to its final decline. We find that the baryon wind carries
$\simeq\!0.010\!-\!0.028\,M_\odot$ of unbound material, proving that the high
mass estimated for the blue kilonova can be achieved. We also find expansion
velocities of up to $\sim\!0.2\,c$, consistent with the lower end of the
observational estimates, and we discuss possible effects neglected here that
could further increase the final ejecta velocity. Overall, our results show
that the magnetically driven baryon wind represents a viable channel to explain
the blue kilonova.
