Blissey's S. Defense caps out at 405 with beneficial nature. Blissey's Defense caps out at 130 with beneficial nature. Chansey's S.Defense caps out at 339 with beneficial nature, with Eviolite it gets about 508 which is more than Blissey's by over a hundred (103 to be exact). Chansey's Defense with beneficial nature caps out at 119, with Eviolite that becomes about 178 which is a bit higher than Blissey's. Chansey gets higher defenses with the Eviolite. Hope I helped :)
Note: Eviolite gives a 1.5 boost to the defenses of not fully evolved Pokemon.
Note: you can not have a beneficial nature to two stats at once.