In my experience with this, HTML text formatting is retained if the signature is copied either from an email you're composing (e.g. sending yourself an email containing your signature and then hitting either Reply or Forward to open up a Compose window) or from an email you're just viewing (i.e. copying the signature before hitting Reply or Forward)...but I can't remember which one.
Whichever way I first did it resulted in the color of the text reverting to black but the 2nd way I did it has worked.
Oh and, in addition to dark blue, italicized text, my company requires us to have an image as part of our email signature (the whole thing doesn't actually look as bad as it might sound) which, as others have mentioned, seems to disappear after a short while.
EDIT: just tested this and formatting is retained when the signature is copied from within a Compose window.