Quote Originally Posted by Halgy
Sorry for sounding so...snob-ish about it. You are right; there are no other ones that are stated as such in cannon, but it is implied a few times that many deatheater pure-bloods are half-bloods. In any case, the possibility of half-bloods being deatheaters is proven by Snape's deatheaterdom.
Quote Originally Posted by Inverarity
I don't recall any such implications
The Blacks blasted Squibs off their Family tree (although this isn't stated in the books, but in the tree drawn by JK Rowling afterwards.) The presence of a Squib in your family is proof that there's some Muggle blood somewhere down the line, just as a Muggle-born must have had a witch/wizard ancestor.

I agree that Snape's parentage wasn't widely known and I don't think Voldemort's was either. Harry taunts Lucius with the fact that Voldy had a Muggle father, but Lucius refuses to listen.