Versioning docx file in Subversion

THe answer for this questions is actually here: http://svn.haxx.se/users/archive-2007-03/1473.shtml

Apply an svn:mime-type of application/msword to all your Word files as an extra precaution. If you use TortoiseSVN, it comes with a script which will let you view the differences between 2 versions using Word's built-in capabilities.

And, very similar to this other question: Is version control (ie. Subversion) applicable in document tracking?

What's wrong with using the built in revisions tool, aka "Track Changes"?

It is part of word, you will just need to store the different versions in source control, the revision tracking should give you your "blame".