harry potter - Did anyone know who the Half-Blood Prince was (Before Book 6)?

The Half-Blood Prince was none other than

Severus Snape

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).

Answer

I could be wrong, but nobody actually knew the term "Half-blood Prince" in the first place as far as I recall. It was a private nickname that Snape made for himself. So the question's answer is "nobody COULD know the identity of that name since nobody knew the name in the first place".

I can see only 5 candidates:

1. 
   

   He clearly wouldn't be likely to reveal his nickname to Muggle-hating "friends" of his (Mulciber etc...).

   
   


2. 
   

   There is zero information that Lily Evans knew it. She could have but we don't have any canon confirmation.

   
   


3. 
   

   Same can be said of Dumbledore. If anyone knew, my money would be on him but as with Lily, there's zero canon evidence that he knew.

   
   



4. 
   

   Pretty much nobody else was close enough to Snape that they would even know his mother's maiden name.

   
   


5. 
   

   That only leaves possible random students who used Snape's textbook like Harry - which was again not a common occurrence (even the poorest ones like Ron Weasley bought their own; and Harry was given the class copy because he unexpectedly was readmitted to the class by Slughorn on short notice. Since Snape was the Potions teacher most of the intervening years, he was not likely to have given anyone his own potions textbook if he ran across it).