From the Super Mario Wiki
A Black Apple is an item in Super Paper Mario. It restores one HP and cures poison. The player can find it on a tree in Overthere Stair. The Black Apple is also the cure for Princess Peach after she eats the Golden Apple. The Black Apple tastes very weird and nasty, and Peach was very irate over the fact that her friends forced her to eat it. The Black Apple is also known as the "taboo fruit".