I've got cast shadows with a ShadowHelper and a Shader working, but the shadow is always a solid non-transparent color.
 
Here's a screen shot. The facial image is the caster and the grass below is the receiver.


Is it possible to make the cast shadow a partially  transparent color?
I would like to make the blue shadow coming behind the portrait appear to just "darken" the grass behind and leave the tiling intact.

If yes, a hint about how to do it would be appreciated.

Thanks