User blog comment:Brandie Janine Adams/Harry's Magical Blood/@comment-119.224.112.251-20120318072053

Think of it like you think of international born children. A child whose parents are French, that give birth to their child in England means the child is essentially an English born French child. Then if the child was to marry an English man and have a child, their child would count itself as half french, even though their mother was also born in England.

Now change all of the England/English's to pure-bloods, half-bloods, magical; and all of the French's to muggles.

A child whose parents are muggles, that give birth to a magical child means the child is essentially a muggle-born magical child. Then if the child was to marry a pure-blood magical man and have a child, their child would count itself as a half-blood, even though their mother was also born magical.