A WebmasterWorld thread reports that URLs added to the Competitive Ad Filter in Google AdSense, are not blocking those URLs.
There are five posts at the thread all this morning reporting the same thing.
I can't believe the timing of your post. I just emailed AdSense Support because this is happening to me as well. One of the domains that is now appearing in AdSense ads on my site has been in my competitive ad filter for over 3 years. And there are others as well. I hope that everyone experiencing this will email firstname.lastname@example.org.
More details on the competitive ad filter at this page.
This is not good, if true. I did not test it myself.
Forum discussion at WebmasterWorld.