I've been complaining lately in various YouTube threads that the old extensions which I would use to block individual channels no longer affect the sidebar (the recommended videos on the right side when you click on a video) after YouTube made some UI changes. After several failed attempts to fix it with uBlock custom filters, I finally got the inspiration to search for a userscript on Greasyfork. And I found one that works so far. I'm sure most of you couldn't give less of a shit about this issue, but for no-lifers such as myself who have YouTube on in the background 16 hours a day, it might help:

https://greasyfork.org/en/scripts/11057-block-youtube-users

Use: once activated, you should see a "B" symbol in the upper right of YouTube pages. Click it (or press ctrl+alt+B) then write the channel name in the blacklist or press the X next to the channel name on screen. You can read the rest on the script's Greasyfork page.