I've noticed that for some reason, after you install Safari 5 on OSX, if you are to do a command where it creates an email out of a file. For instance:
Open a PDF in Preview and you want to email that to someone else, you go to File, and click "Email this PDF" (or similar) It'll create a new email message, but the background of the mail message will be black.
I've noticed this in Omnifocus as well, if I use a shortcut key to create a "To-Do" from another application by using the "Clipping" function, the background of the "To-Do" will be black.
Well, at least in Mail there is a fix.
If you want to keep the email HTML, Command -A will select the contents of the email, Cut it (not copy it), (command x), then repaste it with Option-Shift-Command-V (Paste and Match Style -- this is in the Edit menu). Or... You can change the email to Plain Text (which will get rid of the black box), Plain Text is in the Format menu. Or Command Shift T.
Plain Text is usually better anyway.