As an almost 30-year-old man I'm a little embarrassed to say this but... (I guess minor Harry Potter spoilers ahead if anyone actually cares?)
Harry Potter wasn't the Half-Blood Prince, it was Severus Snape.
The Half-Blood Prince was a name scribbled in a book called Advanced Potion-Making, where Harry learnt a number of dark spells (such as Sectumsempra).
In fact, Harry didn't realise that the copy of the Advanced Potion-Making book was Snape's and thought that he learnt much more from the book than he ever did from Snape (ironic considering the Half-Blood Prince was actually Snape's nickname for himself, Harry wouldn't realise this until later and at the time he did, he hated Snape because he killed Dumbledore).
They are correct in saying that Half Blood means Half Wizard, Half Muggle though. Snape was (IIRC) the only Half-Blood Death Eater.
Yes I've already sent this in as the most unnecessary correction ever.
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