You can preview blending using the Preview view. It's available in both Setup and Animate modes.
It currently doesn't have the option to infinitely loop a preview of blending between two animations while you work though. That sounds useful.
But it falls into the broader stuff we want to be able to do with the Preview view, like preview a sequence of animations.
Currently, you can work the animation, then make Preview view switch between animations. That will show you what the blending looks like. You can also type in your preferred blending duration.
If you meant "blending" as in playing multiple animations at once, you can do that now. Preview view has multiple tracks and you can make it play "run" on track 0 and "shoot" on track 1 and it will preview the resulting running and shooting movement. You can do this while making adjustments to the animations themselves so you get real time feedback.