0 SpA Blissey Flamethrower vs. 252 HP / 4 SpD Aegislash-Shield: 72-86 (22.2 - 26.5%) -- possible 5HKO after Leftovers recovery
0 SpA Blissey Flamethrower vs. 252 HP / 0 SpD Skarmory: 136-162 (40.7 - 48.5%) -- 9% chance to 2HKO after Stealth Rock and Leftovers recovery
0 SpA Blissey Ice Beam vs. 4 HP / 0 SpD Latios: 94-112 (31.1 - 37%) -- guaranteed 3HKO after Stealth Rock
0 SpA Blissey Thunderbolt vs. 4 HP / 0 SpD Mega Gyarados: 82-98 (24.6 - 29.5%) -- 0.2% chance to 3HKO after Stealth Rock
0 SpA Blissey Thunderbolt vs. 248 HP / 252+ SpD Talonflame: 94-112 (26.1 - 31.1%) -- 8.8% chance to 4HKO after Leftovers recovery
Yeah Blissey isn't very threatening with its special moves, outside of 4x effective moves they often barely beat out Seismic Toss and Blissey is simply set up bait for Mega Gyarados unless it has Toxic/T-Wave and BU Talonflame unless it has Toxic (which is harder to fit on if you run a special move because Seismic Toss is still pretty much mandatory).
Chansey outclasses Blissey, not in every single way, but in enough ways to make the latter a worse option in almost every situation.
But that's what I said: "Blissey can even afford to invest some EVs into Special Attack". She can even run a modest nature if she needs to. I personally ran a Blissey with enough SpA to beat Skarmory in two hits with Flamethrower, but even with Ice Beam/Thunderbolt, this Blissey can still achieve some good results:
132+ SpA Blissey Flamethrower vs. 252 HP / 0 SpD Skarmory: 178-210 (53.2 - 62.8%) -- guaranteed 2HKO after Leftovers recovery
132+ SpA Blissey Flamethrower vs. 252 HP / 0 SpD Ferrothorn: 232-276 (65.9 - 78.4%) -- guaranteed 2HKO after Stealth Rock
132+ SpA Blissey Ice Beam vs. 4 HP / 0 SpD Latios: 122-144 (40.3 - 47.6%) -- 48% chance to 2HKO after Stealth Rock
(Blissey wins if Latios has a Life Orb)
132+ SpA Blissey Ice Beam vs. 4 HP / 0 SpD Dragonite: 264-312 (81.4 - 96.2%) -- guaranteed OHKO after Stealth Rock
(doesn't trigger Weakness Policy with SR up, something 0 SpA Blissey can't do)
132+ SpA Blissey Ice Beam vs. 4 HP / 0 SpD Garchomp: 304-360 (84.9 - 100.5%) -- 43.8% chance to OHKO after Stealth Rock
(Coinflip here, yes, but Blissey wins if Chomp has a LO)
132+ SpA Blissey Thunderbolt vs. 4 HP / 0 SpD Mega Gyarados: 106-126 (31.9 - 37.9%) -- guaranteed 2HKO after Stealth Rock and 3 layers of Spikes
132+ SpA Blissey Thunderbolt vs. 4 HP / 0 SpD Talonflame: 180-212 (60.4 - 71.1%) -- guaranteed OHKO after Stealth Rock