The flat spot is where one places the tongue. It helps prevent saliva from draining back into the pipe and causing a gurgle.
The black substance in the bowl is a pre-carbon and allows one to forego the normal break-in of 1/4 bowl, 1/2 bowl, 3/4 bowl then a fill. As Unclearthur said, different manufactures use different formulae but the basic one is yogurt and powdered carbon. There is even one fellow I know that uses grape jelly and dry, powdered tobacco.