Is there an ability to block posts that link to specified sites I know I'll have no interest in seeing?

No, there is currently no feature for that.

You could make a feature request ( if that wasnt aready requested ) here: https://github.com/LemmyNet

Piefed has such filters

It might also be possible for a few Lemmy clients but that's a case by case. There is a "content filter" option on https://www.lemmyapps.com/