Who is Prince of Darkness?
It is most certainly a reference to Ozzy Osbourne. Ozzy Osbourne - Prince of Darkness. 

As for why, this is what I found in  The Bleach Breakdown: The Unofficial Guide
by Issei Handa, Walt Wyman in the Character Breakdown Section, for Rose, emhasis mine.


  Rose
  Long, Blond, wavy hair and seventies hard rock clothes. Seems like a narcissist, and maybe gay. Calls Rabu "Love" (he pronounces it "Ravu", which is one step closer than the typical Japanese pronunciation, "Rabu"). Wishes Rabu would let him read his Shonen Jump. More calm and easy-going than most of the Vizards. He plays the guitar. At one point he mentions "Prince of Darkness's new Album." It can be surmised that he is named after Ozzy Osbourne's legendary first guitarist Randy Rose. Prince of Darkness is an Ozzy Osbourne album.


Thus I consider it mostly an easter Egg about the name Rose.