I don't use Unity but I do use Blender and have worked with facial animation extensively.
Facial animation can be accomplished in one of two ways. The first is through the use of morph targets, or Blendshapes as Unity calls them (in Blender, they are called Shape Keys - every tool has its own term for this thing, annoying huh?). The other way is through good ol fashioned skeletal controls, with bones rigged to the different features of the face.
I prefer shape keys when possible, but sometimes skeletal controls are better. It depends on each specific model. Sometimes, a combination of both is best. Shape keys are nice because you don't have to weight paint or rig the face, you literally just sculpt the vertices into the shape that you want. Saves time.
The Unity documentation mentions blendshapes here.
http://docs.unity3d.com/Manual/class-Mesh.html
As for actually creating these in Blender, you can find lots of tutorials on Shape Keys with a quick google search. This can get you started.
https://www.blender.org/manual/fr/animation/shape_keys.html