Striker Campbell, 31, joins from Queens Park Rangers on a three-month loan, which runs from Saturday February 9th until May 12th.
Midfielder Williamson, 30, arrives at Rovers on a free transfer, signing a permanent deal until the summer of 2014.
After starting out in non-league, Campbell got his big break in professional football when he signed for Brentford in June 2005. After impressing with the Bees, he was quickly snapped up by the Blues in a £500,000 move to Birmingham City.
In July 2007, the striker was on his way to Leicester City in a £1.6m move, but following the Foxes' relegation he went on loan to both Blackpool and Derby County, before returning to the Seasiders in a permanent deal in August 2010.
Thirteen goals in 30 appearances persuaded QPR to make a move for Campbell the following summer, but an injury-hit year restricted his appearances.
In October 2012, Campbell went on loan to npower Championship side Ipswich Town, where he quickly rediscovered his goalscoring form, netting 10 times in a 17-match spell.
Experienced midfielder Williamson started his career with Mansfield Town, making over 150 appearances in five years with the Stags before signing for Northampton Town in September 2004.
A spell with Rotherham United soon followed, before he made the move to Premier League club Watford in January 2007. Williamson would go onto play over 75 games during his two-and-a-half years with the Hornets.
Sheffield United secured the midfielder's services in the summer of 2009, where he would go on to score an impressive 19 goals in 89 appearances.
Last summer, manager Michael Appleton brought Williamson to Portsmouth on a short-term deal, before returning to secure his services here at Ewood Park.