By the time Voldemort had the ability to create any Horcrux's he was no longer permitted within the school. I doubt any follower could either and who's to say that he was even aware of the room?

I mean if you were a person like him, hell bent to find some history, wouldn't you tear that room to shreds?

My theory is the last initial is "Black" but there were so few decent Black family members the odds are I am mistaken.

"B" is just to common a letter to be sure.