This editorial has a list of all the males in the Harry Potter world. Could any of these be the Half Blod Prince?:

"Mark Evans- Unknown- Possibly
Seamus Finnigan- Gryffindor- Halfblood- Possibly
Gregory Goyle- Slytherin- Pureblood X
Rubeus Hagrid- Hufflepuff- Halfblood- Possibly but I doubt it
Terence Higgs- Slytherin- Unlikely
Lee Jordan- Gryffindor- Unlikely
Igor Karkaroff- Not Hogwarts: Durmstrang- Don't think so X
Viktor Krum- Not Hogwarts: Durmstrang- Possibly but I doubt it"

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