Don't you still shatter under Shallow Grave? I thought it was instakill like Culling Blade.
What makes Culling Blade able to kill anything under any condition is because it removes ALL buffs and debuffs from the target before it deals over 9,999,999 damage in a single instance, if your target of course meets the HP threshold. Shallow Grave says you cannot be killed or reduced below 1 HP, and since Ice Blast does not remove the buff it will not shatter you under Shallow Grave's effect. However Ice Blast's debuff typically lasts longer than Shallow Grave, so if you're under the HP threshold when Shallow Grave ends, you bet your ass you are about to become a popsicle.