Blissey is too low tbh, even just for its role on stall it should be B rank. I think people are underwhelmed by it because they use it as a wishpass cleric, which isn't a very good set. If you avoid wishpass, you open up blissey's support movepool and have two slots to dedicated to it (softboiled and seismic toss are a given).
All blissey sets should include stealth rock, without exception. Blissey is the most reliable user of the move in the tier because it beats more hazard removers than any other setter- empoleon does nothing besides phase and has no recovery to withstand being seismic tossed to death, mega blastoise does nothing even with aura sphere and again gets seismic tossed to death, tentacruel and forry also get seismic tossed to death (wouldn't recommend staying in on these for too long because they can set up hazards on you). The others are a little more iffy, you can beat crobat if it doesn't have taunt or is life orb for some reason, if it does have taunt you get into a stally war. Mence depends on your last slot more than anything, which I'll come back to later, but it's again just going to be a stally war. Stall playstyles really really really love having rocks up, and being the best setter of them is pretty huge.
In the final slot there's a few handy things you can go with. Thunder wave is great for hitting common switch ins and helps prevent things setting up on you, but watch out for hera. It becomes kind of scary for work up refresh pidgeot to set up on you because a full para or two puts it in a very bad situation (make sure you bring a backup check though). Another option is the classic heal bell, which is pretty handy because blissey does get a lot of switch in opportunities. There's also toxic, but meh, there's better users.
In the past the reason given for blissey dropping was that although it deals with the large majority of special attackers, they still do their job most of the time- Nidoqueen sets up rocks, Blastoise spins, rose sets up spikes, dragalge sets up spikes. However, if you take advantage of rocks and utility options blissey flips this and takes advantage of the things it walls. For providing reliable rocks, a blanket check to nigh-on every special attacker and having reliable recovery there's almost no reason not to use blissey on stall. It's also very easy to build a specially defensive core around because the things that do get past it are very limited.
Of course, the big thing is that it only works on stall/semi-stall. That said, for being such a central component to what is a decent-ish playstyle right now, it deserves B, considering that Shuckle is there because it's central to an honestly worse playstyle.
