It does what it says, in that it blocks sites. However:

(1) It wouldn't let me manually add, edit, or remove blacklisted sites. In fact, the blacklisted sites didn't even show up on the list. As a result, blacklisted sites cannot be changed or removed and are there FOREVER.

(2) To see if I could somehow get rid of the blacklisted site that I had entered as a test, I removed this add-on, then re-installed it. After I re-installed, not only was the test blacklist site still in effect, the toolbar icon had disappeared completely. And since the icon had been the only way to add sites to the blacklist and it was now gone, the add-on became completely useless.

Too bad ... I would have liked to have seen this one work :-(