Blissey made its first apearance in GSC as an evolution to the already very popular Chansey (Who was present on almost, if not every single serious RBY team). Blissey is not only a special wall, but it can also be deceptively hard to take down with physical hits, if EV'd correctly (which is so simple even a br could). Even in DP, Blissey is still in use while other special walls such as Snorlax drastically dropped in usage.
Blissey's Effect on the Metagame
As said before, Blissey is more like a mixed wall than a special wall. It has the physical resilience to switch into Choice Banded blows from Adamant Aerodactyl, Salamence, Dugtrio (if at full HP) and just stall with Softboiled or Wish/Protect. No physical wall can take so much punishment from special attacks, neither can Snorlax or Regice take this kind of physical abuse. Blissey's presence gave rise to plenty of sets that serve the sole purpose of countering it, things like Boah and McGar wouldn't really be needed without Blissey, seeing as other special walls can be dealt with through more standard offense, while Blissey just laughs at physical blows unless they are extremely powerful, like CB Meteor Mash or STAB CB Returns. Many physical attackers are judged by their ability to get past Blissey or not.
Blissey is mainly used for its walling power, but it can also serve a few other purposes, like Wish-passing, Heal Bell/Aromatherapy, paralyzing things with Thunder-Wave or using Counter on an unsuspecting Metagross or other CBer. But the most popular purpose is really walling and forcing switches on Spike teams, causing damage through switching and the constant Seismic Toss pummel.
Blissey (F) @ Leftovers
Trait: Natural Cure
EVs: 40 HP / 252 Def / 216 SAtk
Bold Nature (+Def, -Atk)
- Ice Beam
- Seismic Toss
This is pretty much the standard Blissey, Aromatherapy heals status while Ice Beam with 240 special attack is a nearly surefire OHKO on any offensive Salamence. This does great on stall teams.
Aromatherapy and Softboiled can be changed for Wish/Protect (note that you can't carry Wish and a status-healing move at the same time due to egg move conflicts) and Ice beam can be swapped for Thunderbolt if you want to counter DD Gyarados instead of Salamence and pack a good punch on Skarmory and other random things.