Is there any reason you couldn't use some kind of box/rectangle collider?
It might not work perfectly in all cases, but if the blue and red objects are fixed, then you could insert a fairly narrow (width <= 1/2 blue/red sphere diameter -- i.e. it sits across the middle of the sphere) box collider (static) between the two objects. You just need to detect the collision and the side/direction it collided from. To be 100% sure it passed through, just make sure it collided with both sides.
This might be helpful in the context of the 'which side did it collide with' issue: http://forum.unity3d.com/threads/one-side-collider.59869/
If you can attack