So - I made a handful of changes here. First, I made sure that your helper "highlight" field gets set to blank at the beginning of the submit rules (I think I left your submit rule disabled in this copy, so check that too). I also set it blank on form load - of course, if you are setting to blank as part of submit, you won't need that, but I wanted your existing test form to work properly.
I added another field and am setting that to the value of the comment field, removing special carriage return characters (referencing this blog post: http://johnliu.net/blog/2011/11/22/infopath-removing-newline-crlf-using-rules.html). Even then, there was a single char remaining in the test field - I decided to work on the assumption that a single character comment could be considered "blank" - hopefully that's ok. Apparently it is a zero-width space, and I can't figure out how to remove that. :)