The 22-year old Polish goalkeeper has penned a season-long loan with the view to a permanent deal subject to an international clearance.
"I am very happy to be at this big club," Sandomierski told the club's official website. "I think it will be a good place for me to improve and I hope to play a part in this big season.
"England is a special country, football is like a religion so I want to be here and I want to do well."
Sandomierski spent part of last season on loan with homeland club Jagiellonia Bialystok and will now further his development at Ewood Park as back-up for current number one Paul Robinson.