If you chained BoM to this card to flip this card's target face-down, Shadow Spell will still affect the target anyway, even if it's flipped face-up by another card effect later on.
But if you don't chain BoM, and just use it to flip the target face-down after Shadow Spell resolve, Shadow Spell will lose its target and just remain on the field meaninglessly.
Anoymous
MangakaJ96