Well it could face issues with Nasty Plotters and some Specs holders. Infernape could boost Focus Blast and OHKO with aid of any residual damage.
24/232+ Blissey gets taken down 28% after SR, 66% after SR + SS, so that's not helping there.
Porygon-Z can 2HKO with a boosted Tri-Attack.
24/232+ Blissey gets taken down with SR and SS anyway.
She won't be able to counter a Specs Lucario, since Aura Sphere 2HKOs.
Bold Blissey can easily switch out to a ghost, I'd rather not stay in and Softboiled because you can easily get critted.
Although situational, but not impractical at all, Specs Flash Fire Boosted Modest Fire Blasts from Heatran can't be stalled out of.
24/232+ Blissey gets 2HKOed after SR anyway.
Gengar sucks more HP with Pain Split and hits much harder with Focus Blast.
Blissey's not beating Gengar anyway unless the Gengar player mispredicts and Blissey manages to get off a Twave on it.
LO Focus Blast from Gengar could possibly 2HKO with SR and one layer of Spikes.
Most Gengars are SubSplit, so Blissey's not beating it anyway.
Shaymin can 2HKO with Specs Seed Flare should the SpD drop happen.
LO Shaymin is better, and that's only if you run Modest Shaymin. Even Subseed Shaymin beats Blissey.
Even some UU Pokemon could cause trouble. Should Houndoom receive a Flash Fire Boost, Blissey can't switch in because Fire Blast can 2HKO, and should it use Nasty Plot, Blissey just can't be used to stop it.
Houndoom is nonesistant, I'd rather be more worried about Infernape because Houndoom is outclassed.
Should Rain happen, Omaster can 2HKO with Hydro Pump with SR.
Most OU RD teams don't even have room for Omastar since they mostly include Kabutops, Kingdra, and Ludicolo + 3 supporters. However, I'll give you that point.
Ambipom can Baton Pass a Nasty Plot and an Agility to any Specs (or LO) holder and make it near impossible to get walled by Blissey.
The most powerful Pokemon are already mentioned above and how 24/232+ Blissey already struggles against them. What else could be stronger?