Campbell should have signed on deadline day so we are led to believe, but we were told that paperwork was presented too late and the move could not go ahead.
At Queens Park Rangers Campbell suffered a loss of form which resulted in him slipping down the pecking order, since Harry Redknapp took charge, he has only made two appearances in the FA Cup.
The 32 year old has however enjoyed a successful loan spell with Ipswich Town towards the end of the year where he scored ten goals in 17 appearances.
Williamson arrives at Ewood Park on a permanent deal; he has signed a contract that will keep him with Rovers until at least the summer of 2014.
Williamson has been without a club since he left Appleton's former employers Portsmouth last month.