I found this interesting explanation of the following children's rhyme online. The explanation follows the verse,
Baa, baa, black sheep,
Have you any wool?
Yes marry, have I,
Three bags full;
One for my master,
One for my dame,
But none for the little boy
Who cries in the lane.
{ The sharing out of wool bales in this rhyme is attributed to the export tax imposed on fleece in 1275. The purpose of the tax was to protect the English textile industry from foreign competition and to increase the value of exports. Woolen cloth commanded a much higher price than fleece, and both were of great importance to England's wealth.
The colour of the sheep in the rhyme must also be of some significance. Black wool is very difficult to dye and would therefore seem to have little application in spinning and weaving. However many shepherds prized black fleece and would keep one coloured sheep in a flock of white ones. When the fibre was prepared for spinning a little of the black was mixed in with the white to produce a light-grey wool. The cloth made from 'grey' wool was believed to be warmer and more weather proof the wool made from white fleece. Unlike the pure black wool it could be dyed without difficulty. Source 2.}