Tomaz mentioned in chat that he couldn't get add a particular blog as an external blog.
The blog in question is http://theridinginstructor.net/?feed=rss2

I had a quick look into it and a bit of debugging makes it apparent that there seems to be an encoding error, Moodle is requested http://theridinginstructor.net/?feed%3Drss2 which is returning the front page of the site rather than the feed.

Not too sure whether this is an isolated incident... will leave that up to the blog expert.

Cheers
Sam