Consider the following damage calculations:

Timid specs Latios draco vs 252 hp / 252 defense / 4 spD bold Blissey:
264-312 damage (simulating a very strong and common special attack)

Timid specs Latios draco vs 252 hp / 252 defense / 4 spD bold Chansey w eviolite:
219-258 damage

Adamant life orb +1 Gyarados waterfall in the rain vs 252 hp / 252 defense / 4 spD bold Blissey:
738-868 damage (simulating a very powerful physical attack)

Adamant life orb +1 Gyarados waterfall in the rain vs 252 hp / 252 defense / 4 spD bold Chansey w eviolite:
538-634 damage

This means chansey is 19.3% more specially bulky, and a little over 29.8% more physically bulky than is Blissey (factoring in Blissey's 10 additional hp).

The only advantage is that Blissey has leftovers. While leftovers are very nice to have, Blissey/Chansey are at many times at full health due to their more than ample opportunities to recover. This nerfs the effectivness of the leftovers and thus Chansey is superior. The only time Blissey is better is on some wish + protect varients in the sand, or if you run ice beam/flamethrower (which is VERY uncommon in high level play).