Catalysis of the reaction: UDP-alpha-D-glucose + delphinidin 3-O-glucosyl-5-O-caffeoylglucoside = H+ + delphinidin 3-O-glucosyl-5-O-(caffeoylglucoside-3'-O-glucoside) + UDP.
UDP-D-glucose:delphinidin 3-O-glucosyl-5-O-caffeoylglucoside -O-beta-D-glucosyltransferase activity