Ozzy Osbourne may have a reputation for being a wildman, but the Black Sabbath rocker still has English manners. This week the Prince of Darkness was spotted walking into Keats karaoke bar in Midtown, where he politely asked the staff if he could use the restroom. “On his way out he was being nice to all the employees and threw a substantial amount of money on the bar,” says the witness. “People were asking to buy him a drink and he politely declined.” Osbourne is reportedly sober after revealing earlier this year he’d relapsed with drugs and alcohol. He was in the city this week with wife Sharon to help launch Black Sabbath’s latest album, “13.” The spy added, “Sadly he didn’t sing karaoke, but we hope he’ll come back.”