This page lists the hosts or subnets for which connections requests will be systematically dropped, even if an explicit rule allows the connection normally. Note that for that list to be effective, the interface(s) to which those IPs are connected must have the "blacklist" options set in the "Zones setup" sub-section.
Do not forget to click on the Apply button at the end of the page once you are satisfied with your settings in order to make your changes effective.