How can I wash a black and white bed blanket so that the black doesn't fade but the white part is brighter?

I have a large comforter blanket on my bed that is white with a black shadow design all over it. How do I wash the blanket so that I can get the white part of it whiter (it's starting to look a bit dingy) but so that the black design doesn't fade? Is there a special detergent or method?