The measurements of android itself are incorrect on a lot of devices. They estimate battery drain instead of measuring it. They do so by checking the uptime of an app and, since Blokada is supposed to always be running, allocate the most drain to Blokada which is entirely incorrect. To measure the actual drain I’d recommend you to check out Betterbatterystats
Additionally you need to make sure that Blokada doesn’t get killed by your system. Why? Because it will try to restart over and over again which will lead to an increased battery consumption. Check the guides and https://dontkillmyapp.com/ for more information regarding that.