Its somewhat interesting to look at blissey for PvP, because it doesnt really look like it will even be good. Yes in gyms its a mild annoyance, able to counter machamp with ZH/DG, as our aim's highest DPS, when you remove the highest DPS being the determining factor of blissey coutner. The adition of the second charge move and the shields also adds a lot to blissey's defensive checks. So many new possibilities open up. Metagross hard resists everything blissey has, same with Registeel (though single bard are hurt by shields, it can still burn through them for the rest of blissey's side), and even Aggron become great defensive counters. Depending on what we see blissey regularly carry, as I feel Pound and hyperbeam will be the standard set for highest neutral damage, as well as not being as hard resisted by metagross, and either dazzling gleam or psychic being carries depending on user preference or meta coverage, in either case many counters open up, for the DG sets, focus blast gengar, BB Charizard and other assorted firetypes possibly frenzy plant venusaur, and eventually Heatran will be a monstrous counter, and for psychic, giratina might see an opportunity here as well as meta stables like Dragonite, Mewtwo with focus blast, Tyranitar, Tanky legendaries lugia and cresselia.
Machamp and other fighting types dont really fit in against blissey oddly enough as regardless of the set blissey has, it will always be carrying a super effective move against them, as well as they dont have the kind of bulk a setting like PvP needs when compared to many other pokemon. Lucario, while not weak to psychic or fairy, doesnt get saved by that as it only bearly lasts longer than machamp because its glass cannon stats ontop of having close combat
Blissey (and chansey) arent the only pokemon that will get hurt from no longer considering only highest DPS counter, but it is possibly the most notable. A lot of common pokemon resist it as well as a few unexpected ones, and when not looking at just DPS, he enormous TDO of these pokemon with those resistances might be able to significantly outlast blissey while also hitting it with higher damage.