I asked a shepherd why there were black sheep in his flock. He said it was to help count sheep. Each black sheep represented a number, for instance 100 sheep. Not sure yet what happens when one white sheep goes missing, but somehow the eye of the shepherd knows by reading the volume around the black sheep.
There are also incredible sheep bridges from the past that helped to count sheep! They were used in Europe and the USA. The photos from 1944 of what must be one of the longest ones called the Verde River Sheep bridge in Arizona, and sourced from the Library of Congress: http://www.loc.gov/pictures/item/az0229.photos.008669p/resource/
I also came across this photo of a shepherd with a stick. Each marking signifies a certain amount of sheep:
Wool Alliance for Social Agency 