This is not true. You can in fact block specific channels. From YouTube support[1]: > On certain pages, such as your Home and Watch Next pages, find a video from a channel that you don’t want recommended to you.

> Click More next to the video title.

> Select Don't recommend channel .

[1]: https://support.google.com/youtube/answer/6342839?hl=en

This is also not true and hasn't been so for years. One can set a preference to "not recommend", but one can not explicitly block any channel.

Depending on your particular "preference constellation's weights" (over which you have no direct control), you can, in fact, be shown videos from that channel again.