....... Except that Porygon Z and Infernape are Glass Cannons. PZ is dead after it kills Blissey. Also, Dusknoir like the free switch in to Hyper Beam. Infernape is ridiculously hard to get in and Blissey can switch.
On Focus Blast, not even Deoxys F can always achieve the 2HKO on Blissey:
756 SAtk Focus Blast (Modest Max SAtk Specs Deoxys F) does 45~53% damage to 714 HP and 405 SDef Blissey (Max, Max+).
That's the strongest Focus Blast in the game. Don't forget the 70% hit rate. Why do you think people forgo Thunder and Blizzard, the awful accuracy.
Lucario is the Fighting type with the highest SAtk at 115 Base.
541 SAtk (Specs Lucario's Max SAtk) STAB'd Focus Blast on 714 HP and 405 SDef Blissey does 49~57%.
That's the strongest Focus Blast in the game, and it's not even a guarenteed 2HKO on Blissey.
So, in conclusion:
Focus Blast =/= teh end-all counter to Blissey
But defensive Calm Minders, aka Slowbro and Jirachi, can beat all variants of Blissey, except CMBliss. Against CMBliss they get into a very long stall war. Jirachi will probably beat Blissey unless it runs Flamethrower and Slowbro has a pretty fair chance of both winning and losing, unless Blissey doesn't carry TBolt, Shadow Ball, or HP Grass (but all variants of a real CMBLiss carry at least one of those three).
To sum this junk up, the only Special Attackers that can get past Blissey and attempt to sweep are CMers who have a reliable recovery move and are not weak to any of Blissey's Special Attacks, which include, standard moves in bold:
Water Pulse
Ice Beam / Blizzard
HP Grass / Grass Knot / Solarbeam (Used in ADV ubers on Groudon teams)
Thunderbolt / Thunder
Psychic
Shadow Ball
Flamethrower / Fire Blast (Used in ADV Ubers on Groudon teams)
HP Fight / Focus Blast
That list is extensive. So even Jirachi is not guarenteed a sweep, the Blissey may be running Flamethrower. I'm not sure about Calm Mind Lucario, Blissey can hit it SE with Flamethrower, but it has STAB Aura Sphere to trash Blissey with. But then again, it lacks a reliable recovery move.
I did not intend on this tl;dr post.