User blog comment:Brandie Janine Adams/Harry's Magical Blood/@comment-219.90.200.67-20120319071555

Lily was a muggle-born. Therefore, that technically makes her a muggle with magical powers, but not magical blood. James was a pure-blood and so, he was a magically blooded wizard with magic powers. Put the two together and you get a half-blood. This happens because the child's traits etc. come from the blood of the parents, not their powers.