Is this change happening when you use the Forward to a Friend link at the bottom of your email or simply forwarding the email through your mail client? If the latter is happening, then what you are seeing is likely a result of your email client rewriting the HTML code into a format compatible with its own editor. Unfortunately, this rewrite of the code can often alter layouts and cause undesirable changes.
As a potential work-around, you may be able to find the section in your email which has the black background color that is showing through when the layout is altered and change that to a while color. This would not prevent the space from being added when forwarding, but it would cause it to be a white space rather than a black space.
And lastly, if it is an issue when using the Forward to a Friend link on your email, then I would definitely suggest giving our Support team a call at (866)-289-2101 and they should be able to assist you further!