The Argentine arrived at Chelsea as an 18-year-old from Chilean side Audax Italiano in January 2008 but he has failed to establish himself at Stamford Bridge and has made just 16 substitute appearances.
Di Santo made his Premier League debut at home to Tottenham last August after impressing in the reserves with seven goals in his first eight appearances.
Blackburn manager Sam Allardyce had been looking to bolster his striking options this summer after Roque Santa Cruz's switch to Manchester City.
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