ill explain it using a great example, the sun.
now, we all know the sun has sunspots, and they appear to be black holes on the suns surface. however, its not that they are actually black, its just that the light produced by the sun is about 400% brighter than those black spots. in reality, the sunspots are ten times brighter than the full moon! so its all an illusion of contrast.
damn, i love the discovery channel. bullshit, i lied. this was from Naked Science on National Geographic.
(thats my source)