Grandupper will make a player's screen go completely black after the battle if they are hit with it and ejected up (8 sturm).
Needs more details. Is it only if they are hit with Grandupper and that's the final attack of the battle? Is it when they are in Grandupper and the battle ends without them having come down yet?
Either. If they are hit with the attack as the finishing blow it will happen, if they are hit with the attack, land, then several turns later the fight ends, it will still happen.