Overview
Natural Cure is important to mention, it's an excellent ability for a wall. Wish/Aromatherapy are also excellent and rare moves. Under cons it's very Taunt prone (which some big special attackers like Yveltal and MMY run) and weak (hazard fodder or set up fodder for stuff that can avoid/cure/is immune to Toxic).
Set
Also mention how Toxic cripples stuff trying to set up. You should also mention that it's quite viable to run Softboiled + Wish on a set, losing the utility of Protect but also preventing Protect abuse by setting up/Roaring Blissey (so that it doesn't heal). With these moves it can heal more reliably than it sometimes does with Wish + Protect but retains Wish passing utility.
Set Details
Calm is also really just needed (as Hack pointed out) to best absorb strong special attacks, especially when it lacks Leftovers.
Usage Tips
One mention of watching out for double switches is enough (both points about switches are pretty much saying this).
Team Options
Remove Ground Arceus (does not check either of those), Aegislash (Blissey covers its main niche decently), Jirachi (lots of redundancy), Ferrothorn (bad physical wall, questionable synergy with Blissey) ,Scarf users (most teams need them anyway and it's not that common for them to be hit with status anyway since they mostly come in to revenge kill), those set up sweepers (once they've been hit with status they probably won't be able to set up again unless they get lucky) and Ditto (if it's even viable then only on HO).
Physical walls are top priority (Lando-T, Groudon and Giratina-O are all good against a lot of physical threats and love Wish support, Giratina works too). Anti-hazard support is really important, as Blissey is affected by all entry hazards and often lacks Leftovers to mitigate it, as well as Blissey being easy to set hazards on. The main support Arceus formes are all good for this though (Water covers Ho-Oh, is a secondary Kyogre check and is a decent check to a number of physical threats although Zekrom needs to be prepared for, Fairy handles Ygod and MMX while helping with Zekrom and Ray, Rock is great for Ho-Oh/Ygod but you need a serious Fighting/Ground check) if you're not already using Giratina-O with Defog. Forry can spin and so can Excadrill (but watch the Fighting/Ground weakness with the latter). Some way to handle Specs Kyogre is needed (Palkia, Waterceus, some other soft checks can be used but they're iffy, while Grassceus doesn't have the best synergy). Calm Mind Arceus make good partners, since status is often the main way to stop them (And mons like Eleceus/Ghostceus can check or soft check some stuff that bugs Blissey too). Tanks like Tyranitar, Heatran and Klefki also make good partners since they love Wish support, and Tyranitar checks Ygod/Ho-Oh while Klefki checks Ygod and can handle subgeo (which can beat Blissey). Mega Mewtwo X is also a good partner, as it threatens prominent stallbreakers such as Gliscor, Dark Arceus and Poison Arceus.
Checks & Counters
Physical threats (add Ho-Oh, remove Terrakion, clarify that Scizor needs Superpower to threaten, replace Groudon with offensive Ground types) > Stallbreakers (Gliscor, Yveltal, CM Refresh or Poison/Steelceus, Sub sweepers, Gothitelle whatever else you come up with that qualifies) > Other Taunt users (defensive Ygod, Skarmer, Sableye, anything that usually has Taunt but isn't meant as a stallbreaker) > Spikers (Klefki gets 3 layers on any set, Skarmer can set up hazards on any set although it has to Roost a lot because Flame does ~40%, Ferro, Forry and Scoli get lots of layers on non-Flame sets) > Rest users (these wall Blissey but they don't prevent its Wish/Aroma utility)
Aromatherapy sounds much cooler than Heal Bell
QC: 1/3