lol Snape and Voldemorte would kill you slow if you told them they were the same as mud-bloods.  Muggle borns are born of two muggle parents the fact they are wizards was a fluke, like stumbling across a winning lottery ticket, though the slytherins would say that it occured too often.<br/><br/>Half bloods ahve a wizard parent, so there is a direct connection to magic, hence the presence of their power.<br/><br/>While there may be, he is in a house with a bunch of people whose parents will try to kill him if his heritage is made known.  It will be intresting to see how he handles it.