It means black or covered in pitch. Really, thats it. If you are a fan of a certain reality TV show in the good old US of A, you may think it is some cool insider term of the music biz that means something like "singing off key".
But not really. It is a made up word coined by a couple of doofuses that couldn't think of a more professional-sounding way to say, "You didn't sound particularly good to me when you sang that song."
