The Championship side are on the verge of a new signing.
The 20-year-old was on loan at MK Dons for the first half of last season, where he failed to score in 15 games, despite registering two assists on his debut against Rotherham United.
Gallagher then returned to his parent club in January and he has made 20 appearances for the Saints to date, striking twice.
However, despite scoring against FC Twente in a recent pre-season friendly, Gallagher has been deemed surplus to requirements at St. Mary's.
An academy graduate, Gallagher holds the distinction of having scored the winner in the Under-21 Premier League Cup final for Southampton against his potential new employers.
While his record leaves much to be desired, his performances have convinced Blackburn boss Owen Coyle to make a move for the imposing 6ft 4in forward.
After losing 4-1 to Norwich City, Coyle has pinpointed the need of a new striker in his ranks, and has identified Gallagher as a prime target.
A player with considerable aerial ability and a keen eye for a pass, Gallagher would seamlessly fit into Blackburn's and Coyle's system of play.