<b:if cond='data:comment.author == "PUT_USERNAME_HERE"'>
and
<b:if cond='data:comment.authorUrl == "http://www.blogger.com/profile/01234CHANGETHIS56789"'>
But the easiest way, and least error prone, is:
<b:if cond='data:comment.authorClass == "blog-author"'>
After realising this, I updated my How to Hightlight author comments in Blogger hack to use this easier method.