Pluto is roughly five billion km from the Sun, and from that distance the Earth and Sun are so close together they might as well be sitting in the same spot.

So when the probe swept past Pluto, it was heading very nearly straight away from the Sun, and when it looked back to take the images comprising this shot, the Sun was on the other side of Pluto…  when means New Horizons was seeing the dark, unlit side of Pluto.  A crescent Pluto glows magnificently in a mosaic by New Horizons; twilit by the Sun you can see mountains and maybe, just maybe, a cloud.