The Newsblur Safari Helper app, which intercepts “feed:” URLs and redirects them to newsblur.com, doesn’t work on Mountain Lion.
I’m pretty sure this is because the app is not signed, and Mountain Lion no longer launches unsigned apps unless you ctrl-click on them and open them from the context menu.
For more info: http://support.apple.com/kb/HT5290
It seems that someone at Newsblur will need to join the Mac Developer Program and acquire a Developer ID, so the app can be signed. (It does not need to be distributed through the Mac App Store.)