Sivir Spell Shield (Gameplay)

When playing against Varus as Sivir, I noticed that spell shielding Varus's ult is nearly impossible. Varus's blight passive procs in priority to his ultimate- this makes it so that Varus can prevent Sivir from spell shielding his ult by simply appling one stack of Blight and then using his ultimate (or any other ability for that matter) to detonate blight. For example, if Sivir has her spell shield up, then Varus auto-attacks her one time, then uses his ultimate, Sivir will NOT take damage from Blight, but will still be snared by Varus's ult. I played a ranked game and noticed this, then took it into a custom game with a friend, sure enough, when Sivir spell shields with no stacks of blight on her and is ulted, the ult is blocked. As soon as even one stack is put on her, she cannot block Varus's abilities.