I think what you are seeing is the background of the Windows MediaPlayer. It is possible that you could see the default black background as the WMP begins drawing the first frame of video. Normally, this is not an issue because the default background of DirectRT is black also. I just checked though and WMP does give you the option to change the colour of the background. Check this out in WMP:

tools > options > performance > advanced > video border color > change

In case it helps, this is a Windows XP machine running Windows Media Player 10.4019.