If you remove a site from the blocked sites list, there can be a delay of up to 5 minutes while the Winston DNS cache refreshes itself. If it takes longer than this, check to see if your router is caching DNS entries. If you need access to the site right away, you can temporarily whitelist it. You can also flush your router's DNS cache to speed up the process.
Note: It is recommended that you disable DNS on your router as Winston already contains a DNS server. This can free up resources on your router and make it and your entire network run a little faster by avoiding an unnecessary DNS lookup. For more information, visit this article.