252+ Atk Aegislash-Blade Head Smash vs. 252 HP / 0+ Def Blissey: 770-906 (107.8 - 126.8%)--Guaranteed OHKO
So yes, it is possible, if Aegislash had 0 HP IVs, because Blissey has a max HP stat of 714, and Aegislash blade has 293 HP, and Head Smash does exactly half the damage dealt.
Hope I helped!
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