maybe if it was a magus Dead Apostle like Nrvnqsr or Roa it would make them unable to use their magi powers but they would still have their Dead Apostle abilities, but someone like Merem who don't use magecraft it probably wouldn't work.
bc he doesnt have magic circuits so he's physically unable to use any kind of magecraft
Origin bullets mess up the magic circuits, and iirc servants do have magic circuits, like when they mention Artoria having a magic core instead because she has dragon blood.
On Casters, probably.
On other classes, only if they use magic. Like Cu doing runes or Emiya doing trace on.
Because he nearly died there, I suppose
Sakura also just turned out that way because she was a Lesser Grail, who then fused with Avenger. I think blackening only occurs with spirits, so humans just die.