Simply put, it does NOT work on all sites. Good case in point is www.aol.com. It does not stop any flash video content from starting up the minute I hit the home page for AOL. I've reviewed the video for using it and also read the instructions and FAQs. But it simply does not work on AOL no matter that the URL list is totally blank. And AOL is not the only site where I've had this problem with Flash Block (Plus).
Fortunately the older competing add-on, Flashblock, does work on ALL websites even if it is a legacy product.