Right-back Yan Valery put the hosts ahead at the near post just before the interval.
Matt Targett doubled the lead with a 25-yard free-kick nine minutes into the second half.
Lewis Mansell hit the bar late on for Rovers, who slipped to their first defeat in 10 league and cup matches.
Southampton: 11 Dec v Wolves (H) PL2
Blackburn: 11 Dec v Reading (H) PL2