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.

You can block from recommendations but there's either a fixed length with the oldest bumped off or it resets. I have to reblock stuff all the time.