Det är en fråga om definition, och hearkens back liknar "En droppregel" som brukade vara välkänd i USA. I grund och botten, för att vara en renblod, måste du komma från magiska familjer på båda sidor, inte bara två magiska användande föräldrar. Omvänt något muggle blod gör dig i bästa fall halvblod.
Från Pottermore:
"The term 'pure-blood' refers to a family or individual without Muggle (non-magic) blood. The concept is generally associated with Salazar Slytherin, one of the four founders of Hogwarts School of Witchcraft and Wizardry, whose aversion to teaching anybody of Muggle parentage eventually led to a breach with his three fellow founders, and his resignation from the school."
Ingen av familjerna är riktigt rentblodiga, som Dumbledore påpekar i Beetle the Bard
"So-called pure-blood families maintain their alleged purity by disowning, banishing, or lying about Muggles or Muggle-borns on their family trees. Then they attempt to foist their hypocrisy upon the rest of us by asking us to ban works dealing with the truths they deny. There is not a witch or wizard in existence whose blood has not mingled with that of Muggles..."
Harry Mors familj är inte en erkänd trollkarl, så han har muggle-blod. Varje muggleblod med trollblod definierar honom som ett halvblod.
Att gå rakt på hästens mun, här är JK Rowling om ämnet:
(Obs! Det här är från Harry Potter Wiki, och jag har inte kunnat underbygga det som en faktisk JKR-citat än.)
The expressions ‘pure-blood’, ‘half-blood’, and ‘Muggle-born’ have been coined by people to whom these distinctions matter, and express their originators’ prejudice. As far as somebody like Lucius Malfoy is concerned, for instance, a Muggle-born is as ‘bad’ as a Muggle. Therefore Harry would be considered only ‘half’ wizard, because of his maternal grandparents. If you think this is far-fetched, look at some of the real charts the Nazis used to show what constituted ‘Aryan’ or ‘Jewish’ blood...the Nazis used precisely the same warped logic as the Death Eaters. A single Jewish grandparent 'polluted' the blood, according to their propaganda.