Old school slang for common folks back in medieval times. Those of low class work in hard labor (agricultural) and spending majority of the time being exposed to the sun; so they tan. Where as the high class folks do nothing, but stay inside their homes; getting paler. So pale that their skin become--as some say like alabaster, that you can see their blue veins through their skin. Thus the word "Blue Blood".
by cinnamonbon June 24, 2009