According to Apple Knowledgebase article TS3385, after updating to either Mac OS X 10.6.4 or Safari 5 ( with Leopard or Snow Leopard), the Apple Mail application may display some emails automatically generated by applications may be displayed with a solid black background, making the text unreadable.  Apple is working on a fix, but until it is released their workaround is clicking in the black message body area, then selecting “Make Plain Text” from the Format menu.