Blissey is only in UU, but Chansey is in OU. I know Chansey is more bulky with Eviolite, but Blissey has usable Special Attack, and Leftovers.
Eviolite is the only reason anyone would use Chansey over Blissey. Eviolite makes Chansey a lot bulkier than Blissey, with both more Defense and Special Defense. Loss of Leftovers isn't that much of a problem when you've got Soft-Boiled anyway.