A change caused by the taint of an unknown curse.
Blackening primarily affects Servants, and corrupted Servants will have the personality aspect of their alignment inverted.
Also, because they receive a physical body, they become more powerful.
It’s a bit like the people resurrected as villains by evil organizations in tokusatsu shows, but since they come back with a different name and a whole new design, it’s probably best to think of them as entirely separate characters.

Fate/side material: Fate Encyclopedia