Hmm... I'm not getting the results I would expect. I think this setup is too dependent on the incoming diffuse light needed to detect where there are shadows, and the Gain node is not giving a sharp enough transition, so we are actually getting a blend of the two inverted textures, hence the greys.
puts thinking cap back on

"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)