To block by category, go to the Category section under Blocking in your account settings.
Anything enabled/disabled here takes effect immediately on the blockerDNS side. However, you might continue to see a relevant domain being blocked/allowed for a little while after making a change due to the DNS cache held by your device's operating system and even your browser. You can clear caches manually if you need a block/unblock to be reflected right away. If not, caches will expire on their own after a few minutes.