Blender version 17:2.78.agit1.e8299c8-2.parabola2 was superseded by blender 17:2.78.a-3.parabola1, but pacman sees this as a downgrade.
For this reason users of the old blender are required to allow downgrades on their next update:
pacman -Suu
Blender update - Manual intervention required
2016-12-29 - Isaac David