The Half-Blood Prince was none other than
Pursuant to this question:
Was there anybody who knew the Half-Blood Prince's true identity? (Before the events in HBP)
It doesn't strike me as something that was well-known (or known at all).
When the true identity of the Prince was revealed in Harry Potter and the Half-Blood Prince (Chp. Twenty-Eight, Flight of the Prince); Harry is alone with the Prince, and so no one other than the two of them know.
It seems also that the information was kept well secret; no one made the connection between the book and any spells invented in it to the creator (the Prince).