Actually, Blissey was better than Chansey in some ways.
So, Chansey's only common use was Eviolite, but it could get rid by Knock off and opponent will kill it easily, but Blissey? It could access with Heavy-Duty Boots, Leftovers and gives it good recovery each turn and could even paired with Live Orb due to that it could use Serene Grace + Tri-Attack and which was common in china
Chansey got lame speed and Blissey's speed could outspeed some Pokemon such as 0 Spe Crawduant and has a faster chance to recovery from all those stabs from it, and chansey's speed was so slow that makes it can't Stuat others and Recover Faster. Blissy instead for being wall,could also be a Tank, due to its Specisl Attack wasn't too low and could paired with Serene Grace + Tri-Attack which was much more better than Chansey who could only be a Tank.
Hope it helps!