Even though when you're in the settings the preview is black, I would go
through the process again to make sure it is indeed set to black: Tools >
Options > Mail Format > Signatures > Edit > Font > Color... and set the color
again. If that doesn't do it, make sure you didn't accidentally click the
font color to red at your new message beFORE you inserted your signature--if
you accidentally clicked the box while in the new message, when you add your
signature it will be added in the color the button happened to be set on. If
that doesn't work, we may need further investigation.