If I could find anything blacker than black, I'd use it.  ― J. M. W. Turner

Shadow is the obstruction of light. Shadows appear to me to be of supreme importance in perspective, because, without them opaque and solid bodies will be ill defined; that which is contained within their outlines and their boundaries themselves will be ill-understood unless they are shown against a background of a different tone from themselves.  ― Leonardo da Vinci