When Safari 5 was released a small bug cropped up where Mail messages including attachments that were generated from scripts and other programs would display with black backgrounds, causing the mail to be unreadable. So far there have been two small utilities released that provide workaround for this bug.
It appears the problem is in the CSS properties associated with attachments, which should be easily addressable by Apple in an upcoming update; however, until then there is an additional fix available for those who use the program "Mail Attachments Iconizer", which is an add-on to Mail which customizes how attachments are displayed in Mail messages and fixes a number of odd problems that people may experience with the default attachment handling behavior in Mail.
The developers have recently updated their program to address the black background bug in Mail, so if you have this application installed, be sure to update to version 2.1.10 to fix this bug. For people who do not use Mail Attachments Iconizer, the program is $15 for a single license.
As mentioned in a knowledgebase update from Apple, the following workarounds can be used to address the problem if you do not care to install or pay for a third-party add-on:on the matter and a
Choose "Make Plain Text" from the "Format" menu (or use Command-Shift-T) and then reverse this if you wish to add additional text formatting.
Select all content of the message and use Command-X to cut it from the message. Then create a new message and use Option-Shift-Command-V to paste it and match the style so no formatting in the automatically generated email will be lost.
Both the update to Mail Attachments Iconizer and new licenses can be found and purchased at the Mail Attachment Iconizer website.
Thanks to MacFixIt reader "goodwinltg" for bringing this to our attention
Update: There is another utility out that also provides a workaround for this bug. "FixMailText" is a small freeware utility that will do the same thing, but does only this. For people just looking to avoid this bug you may find FixMailText to be a better option; however, you may also find the attachment enhancements provided by Iconizer to be useful as well.