The 31-year-old Campbell was due to sign for Rovers on the final day of the January transfer window, but the move was not completed in time.
However, the player has now switched to Ewood Park on a temporary deal, which officially starts on Saturday and runs until 12 May and includes a recall clause, which can be activated after 28 days.
Campbell could be in line to face Ipswich, with whom he enjoyed a loan spell earlier in the season, scoring 10 goals.
Town were reportedly keen to bring the former Birmingham, Leicester and Blackpool forward back to the club, with Hull also interested.
Experienced midfielder Williamson, 30, has signed a deal with Blackburn until the summer of 2014. He 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.