Another thing: the Specular_Color looks like black, but it might not be quite black. There should be no specular reflections on the background, so I would set the Specular_Value to zero to make sure of that. You can also delete the lower Color_Texture node.

"If I were a shadow, I know I wouldn't like to be half of what I should be."
Mr Otsuka, the old black tomcat in Kafka on the Shore (Haruki Murakami)