Yep, not a 2HKO, but can make a big dent on her, and finish off with some phys-move/ forces her to heal and switch to a usefull counter.True, but it's still only one side of the defense spectrum. If it were both, it'd be comparable.
252SpAtk, Neutral Nature, No weather Fire Blast against Standard Blissey: 18.02% - 21.22%
252SpAtk, Neutral Nature, Sunny Day Fire Blast against Standard Blissey: 27.03% - 31.83%
252SpAtk, Positive Nature, Sunny Day Fire Blast against Standard Blissey: 29.65% - 34.74%
252SpAtk, Positive Nature, Sunny Day, Solar Powered Fire Blast against Standard Blissey: 44.33% - 52.18%
Not even a 2HKO.
Wanna calculate it for cresselia too? =P
PS: if this can ake 52% against Standart blissey with Stak without SE isnt bad, and can kill many things
PSS: Even if its defensive, Life orb/choice specs will kill her