In the scene below it is simple model on a plane with a single lamp set off camera high right, with no AO.  In the top picture the shadow colour is set to fully black.  When the shadow colour is changed to a shade of grey it reveals details on the dark side of the model which would not be receiving any light.  The main face of the wall in darkness retains the full black colour but the window type areas do not.  The small bottom thumb nail further highlights this.  Why would these areas of the model that would not be receiving light and therefore no shadow casting still somehow be influenced by the shadow colour? 

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