@KoiwiGal - Black bread mold is one of the most common examples of this kind of fungus. Everyone has seen it before, it's kind of greenish-grey, and goes black when it gets older.
Although it's usually not that pretty, it's still really useful. It doesn't seem like it to us, because it helps to spoil our food. But if those oranges are lying on the ground, under a tree, this mold helps to decompose them and turn them into soil.
It's one of the things I teach my kids in school. You should also try to see where everything fits in the ecosystem before just dismissing it as a "bad thing". Even mold has its place and its purpose.