While blending poses from the pose library, press {key F} to flip the
pose.
Since rBc164c5d86655 removed the "Flip Pose" checkbox, there was no more
way to blend poses flipped.
Note that this is a bit of a stop-gap measure. The goal is to have a
bidirectional blend slider, where dragging to the left flips the pose,
and dragging to the right blends it normally.