In that "Stop Staring" book there's a section on that. The author calls this "half shapes", and it's a technique where you can get round movements even though blendshapes are linearly interpolated, like you noticed. It's the combination of two blendshapes: a main blendshape for up-down movements, for example, and a…
Morphs and BlendShapes are great, you have precise control over the shapes you need so you don't have the eyeball clipping through the lids, but they can be tech heavy and morphs aren't always a good idea for every face in every game/engine. Morphs can also be a problem with some eye sockets if they aren't modeled in…