If you’re experiencing an issue in which you see a black background in Mail messages, rendering text unreadable, download and install Safari 5.0.1. The update fixes the issue, according to a new Knowledge Base document from Apple, which states:
“After installing Safari 5 (with Mac OS X v10.5.8, Mac OS X v10.6.2, or v10.6.3), or after installing Mac OS X v10.6.4 (which includes Safari 5), you may see a black background instead of readable text in Mail messages that are generated by some applications. […] Resolution: Download and install Safari 5.0.1 or later.”