Originally posted by NeoStrider
otherwise the 'black hole matte' is actually a material. it's called the 'use background shader' and when applied looks green in the viewport. it doesn't show up in renders but shows up in alpha channels, and it still receives shadows from the other objects in the scene.
The black hole matte is actually in the material attributes, blinn, lambert, phong etc etc, its in the matte opacity tab in the drop down menu.
You also get the option to overide cirtian materials in the render layers so that it does this for the layer its rendering.
You could use the surface shader, but the matte opacity is just the job for this.
"No pressure, no diamonds" Thomas Carlyle