The tinyMCE loads in a stylesheet that controls the background color. Usually the file name is content.css or content.min.css
If you can find this file in your particular configuration and set the background color of the body element to black, you should be set.
For example: http://www.tinymce.com/tryit/basic.php
The style sheet is located at:
http://tinymce.cachefly.net/4.0/skins/lightgray/content.min.css