Blissey v. Chansey depends on the game you're playing and the battle format you're playing.
Firstly, it is true that Eviolite Chansey's Defence is better than Blissey's in all practical scenarios*. This is because Eviolite boosts the holder's entire Defence stat (with EVs, IVs, and all other factors included in the calculation), not just its base stat.
If we compare Chansey and Blissey with the set they both typically use in competitive (252 Def, Bold, 31 IVs), their stats would reach 119 and 130 respectively. However, with Eviolite's 1.5× boost, 119 reaches 179 after rounding. This is approximately 37% better than Blissey. Eviolite Chansey also has higher Special Defence than Blissey in general. More info on stat calculation is available on Bulbapedia, and you can try some calculations yourself with this tool.
Not all is lost for Blissey, though. In any context prior to Gen 5 (where Eviolite is unavailable), Blissey is obviously favourable. Additionally, with the introduction of Heavy-Duty Boots in Gen 8, the fortunes flipped back in Blissey's favour. This is because immunity to entry hazards is more valuable than Chansey's bulk over the course of (6v6 singles) matches. Doubles formats that are less switch-heavy may still favour Chansey out of the two.
* What are the impractical scenarios where Blissey's Defence is better than Chansey's? One example is when Eviloite Chansey and Blissey both have absolute minimum Defence investment (0 EVs, 0 IVs and a negative nature). In this case, Blissey would beat Eviolite Chansey 22 points to 20.
