  The 2017 detection of the inspiral and merger of two neutron stars in
gravitational waves and gamma rays was accompanied by a quickly-reddening
transient. Such a transient was predicted to occur following a rapid neutron
capture (r-process) nucleosynthesis event, which synthesizes neutron-rich,
radioactive nuclei and can take place in both dynamical ejecta and in the wind
driven off the accretion torus formed after a neutron star merger. We present
the first three-dimensional general relativistic, full transport neutrino
radiation magnetohydrodynamics (GRRMHD) simulations of the black hole-accretion
disk-wind system produced by the GW170817 merger. We show that the small but
non-negligible optical depths lead to neutrino transport globally coupling the
disk electron fraction, which we capture by solving the transport equation with
a Monte Carlo method. The resulting absorption drives up the electron fraction
in a structured, continuous outflow, with electron fraction as high as $Y_e\sim
0.4$ in the extreme polar region. We show via nuclear reaction network and
radiative transfer calculations that nucleosynthesis in the disk wind will
produce a blue kilonova.
