Keeley, 51, worked in the same role under new Rovers boss Michael Appleton at Portsmouth and represented Southend, Brighton, Oldham, Colchester and Stockport during his playing career.
Bobby Mimms held the position of goalkeeping coach at Ewood Park until he was dismissed along with assistant manager Eric Black and coach Iain Brunskill when Henning Berg was sacking as manager last month.
Keeley was originally brought in to coach the academy goalkeepers at Pompey but stepped up to the senior side in 2010.
Head of recruitment Luke Dowling is also expected to join the Championship club.
Pompey caretaker manager Guy Whittingham said: "They have both done a great job at this club and I understand why they wanted to take such a good opportunity."