In the program's main window you can see two columns: one on the left where you can see the allowed sites, and another on the right with the blocked sites. Any web address or word you enter in the right column will be completely blocked, making it impossible to browse or search for it via Google, Bing, Yahoo, etc.
You can set a series of passwords to access the program or uninstall it. This makes it nearly impossible to breech the program's security ... as long as a different browser isn't used.
BlockAllow is a pretty limited application. On one hand, it does its job well and correctly blocks the pages you indicate, but on the other hand it's easy to get around its protection by simply using another browser like Mozilla Firefox.