Hi,
A tween and physics behaviour (bounces/etc) can't work together, since a tween is like fixed animation that will forcefully go its own way in spite of external forces. So your blocks actually collide, but the tween just make Unity ignore that and move on.
What you could do, is use a callback for collision (like OnCollisionEnter) to determine when a collision happened, and stop the tween so that the physics engine can replace it.
Cheers,
Daniele