With the addition of Psyshock, stronger fighting types, and dangerous set-up sweepers, Blissey has a much harder time functioning in this generation than any other generation yet. Many players would also rather opt for a more bulky Eviolite Chansey. Despite all of the major problems and competition this pink fatty faces, she has been blessed with a few notable features. Blissey gets one thing that Chansey doesn't, leftovers recovery. New Wish Mechanics allow Blissey to fully heal nearly any Pokemon since Wish now heals 50% of the users HP. It seems like no matter what new generation is introduced, Blissey always manages to find a proper spot in OU.
Blissey @ Leftovers
Ability: Natural Cure
Evs: 252 HP/252 Def/4 SpD
-Aromatherapy/Heal Bell/Stealth Rock/Flamethrower
Blissey has not received very many upgrades over the past years. Her bread and butter set still remains nearly the same. This set aims to support the team with status inducing moves, status healing moves, and/or Stealth Rock. Softboiled provides reliable recovery and Seismic Toss deals reasonable damage to anything that isn't immune. Flamethrower can also be used to eliminate enemy Ferrothorn and Skarmory who think they can come in on Blissey. The choice between Toxic and Thunder Wave depends on what play style your team is. If it's stall, Toxic is the superior option to slowly break down other Pokemon. Thunder Wave is especially useful if your team is offensively oriented to slow down opposing Pokemon and make it easier for your team to sweep.
EVs and Nature are crafted so Blissey can take maximum punishment from physical attacks that aren't Super Effective STAB fighting moves such as Close Combat. Item choice is and always will be Leftovers, it's the only thing that Blissey still has over her little sister Chansey. You may also choose to run only Flamethrower as the attacking move since Blissey also has a higher SpA stat than Chansey.
Blissey @ Leftovers
Ability: Natural Cure
EVs: 252 HP/252 Def/4 SpD
Blisseys colossal HP stat is a godsend for her and it allows her to effectively utilize Wish to support her team. Thanks to the new Wish mechanics, she can easily heal nearly any Pokemon to full health. Wish is obviously the sets staple, and the choice between Softboiled and Protect is up to you. Softboiled allows you to have more healing PP and give you more chances to heal your own Pokemon, although running out of PP is not much of an issue. It also allows you to instantly heal rather than use Wish+Protect in important times. If you would rather opt for Protect, you can scout what your opponent wants to do. However, it can leave a free turn for your opponent to switch out to a counter or a free set up turn.
Durable physical walls such as Skarmory and Gliscor make good team mates. Pokemon that can switch in on powerful fighting type moves are easily supported by Blisseys amazing Special Defense stat. Blissey can function in nearly any stall or balanced team and provide support in various ways such as Stealth Rock support.
An unreliable but surprising set using Counter can be used. Most attacks aimed at Blissey will be Physical which gives you many chances to use Counter. Unfortunately once Counter is used, the surprise factor is completely lost and you probably wont be able to use it again that battle. Calm Mind can also be considered so it can boost its special defense even more and give it some kind of chance of offense. Many things can force Blissey to switch out, so Calm Mind is not advised. Blissey also gets the coverage moves Thunderbolt, Shadow Ball, and Ice Beam, all of which can be used effectively. Blisseys somewhat usable special attack stat can be exploited to catch those pesky Dragon or Flying switch ins. However, Seismic Toss usually provides the most reliable and consistent damage output.
Checks and Counters:
Blisseys Achilles heel is fighting type moves such as Close Combat and Focus Punch. The addition of stupidly strong fighting Pokemon such as Meinshao and Conkeldurr will give this pink blob a tough time. Conkeldurr does not fear status moves from Blissey since it receives an attack boost with Guts and can return fire with a STAB super effective Drain Punch. Whimsicott can Taunt Blissey and gain a lot of HP from Leech Seed. Taunt users in general can totally destroy Blisseys walling ablility. Trick users such as Rotom-W can cripple Blissey by giving it a choice item and locking it in one move. Several Pokemon can come in and set up in Blisseys face, especially if the Pokemon sits behind a Sub. Max HP Jirachi subs are not broken with Seismic Toss and can set up or Paraflinch Blissey to death. Be aware of Psyshock coming from the likes of Latios and Reuiniclus, which actually hits Blissey on the Physical side of the spectrum. Reuniclus can strike with Psyshock and isn't affected by status with the ability Magic Guard, two things that Blissey really hates.
Blissey receives Healer as her Dream World ability. Healer has a chance to heal a partner Pokemon by 30% in a double or triple battle. This ability obviously serves no purpose in a single battle.