Nothing is blocking it. It's just the Sun shinning from another angle. One side of the Moon always faces us, but since it rotates around the Earth.
One side never constantly faces the Sun.
And then, of course, there is the ever-popular web searach: csep10.phys.utk.edu...
The Moon appears to go through a complete set of phases as viewed from the Earth because of its motion around the Earth, as illustrated in the
following figure.
as the moon rotates around its orbital path the sunlight coming from the sun will strike it at different angles creating the phases of the moon. as
for why we always see the same side of the moon, that is because the moon's rotational and orbital period are the same (27.3 days) cancelling out a
visible rotational effect.