Blending in Mecanim is definitely supported but it doesn't use a unity "Avatar" (correct me if I am wrong). When exporting to unity, the SkeletionData object will let you create an animator that will work with all your animations. You can add animations (or blend trees of animations) to states and they will mix properly during a transition. If you want to mimic tracks, use mecanim layers. For example, you could have a layer for the legs and one for the arms.