According to Scott Wheeler (the maintainer of TagLib, a popular tagging library) writing "0" instead of "0xff" when no Genre is set causes programs to incorrectly display "Blues" as a genre. It should of course be empty.
http://mail.kde.org/pipermail/taglib-devel...ary/000565.html
I just noticed MP3tag v2.38 edited files without a genre show up with genre Blues. Is this a bug in MP3Tag?