Allardyce has been looking for more options up front and 20 year old Di Santo will certainly offer the club that.
The Argentinean made only a minimal impact on the first team at Stamford Bridge after he was signed from Audax Italiano in 2008. But he will stay with Blackburn until January.
Rovers have the option of extending the deal should the player impress, but that will depend on whether Chelsea are willing to let him stay.
Di Santo could make his first appearance for Blackburn in Wednesday's friendly game with Hibernian.