This is a good question. To say something is a case of black and white is to say something is clear and beyond a shadow of doubt. In other words, there is no ambiguity and confusion.
The case that takes center stage in To Kill a Mockingbird is such a case. It is absolutely clear that Tom Robinson is innocent. It is beyond a shadow of doubt. Not only is Tom a good man with unassailable character, but he is also handicapped. He could not have beaten Mayella.
On the other hand, Mayella's father, Bob Ewell, has a temper and is a drunk. He is the one who beat her. Tom even witnessed it. In this sense, all evidence says that Tom is innocent and Bob is guilty.
That said, there is a deeper meaning to the sentence. In Maycomb, a racist town, there is no chance that a white jury will say that a black man is innocent, no matter how much evidence there is, especially against a white man. In this sense, this case is really black and white. More specifically, it is white versus black.