I think the issue there is probably that the block applies to what's in the (some.domain) next to threads. For non-link threads, it's hidden, but it's the instance the thread was created on. But for threads that link to an article, image, video, or anything else, it's the domain the link points to. So a thread linking to a youtube video will have youtube.com as its domain. And only blocking youtube.com will hide it, not blocking the instance the thread is from.
It think ideally both, the target of the link and the owning instance of the thread, should be accounted for by the block functionality. But I don't think that's currently the case.