  The decline in abundance of Lyman-$\alpha$ (Ly$\alpha$) emitting galaxies at
$z \gtrsim 6$ is a powerful and commonly used probe to constrain the progress
of cosmic reionization. We use the CoDaII simulation, which is a radiation
hydrodynamic simulation featuring a box of $\sim 94$ comoving Mpc side length,
to compute the Ly$\alpha$ transmission properties of the intergalactic medium
(IGM) at $z\sim 5.8$ to $7$. Our results mainly confirm previous studies, i.e.,
we find a declining Ly$\alpha$ transmission with redshift and a large
sightline-to-sightline variation. However, motivated by the recent discovery of
blue Ly$\alpha$ peaks at high redshift, we also analyze the IGM transmission on
the blue side, which shows a rapid decline at $z\gtrsim 6$ of the blue
transmission. This low transmission can be attributed not only to the presence
of neutral regions but also to the residual neutral hydrogen within ionized
regions, for which a density even as low as $n_{\rm HI}\sim
10^{-9}\,\mathrm{cm}^{-3}$ (sometimes combined with kinematic effects) leads to
a significantly reduced visibility. Still, we find that $\sim 1\%$ of
sightlines towards $M_{\mathrm{1600AB}}\sim -21$ galaxies at $z\sim 7$ are
transparent enough to allow a transmission of a blue Ly$\alpha$ peak. We
discuss our results in the context of the interpretation of observations.
