Thanks! For anyone that wants something that will block posts and comments do the following in uBlock Settings -> My Filters:

your.instance##.post-listing:has(*:matches-attr(href=/.*@blockedinstance\.domain/))
your.instance##.comment:has(*:matches-attr(href=/.*@blockedinstance\.domain/))

In this case from blahaj to hexbear:

lemmy.blahaj.zone##.post-listing:has(*:matches-attr(href=/.*@hexbear\.net/))
lemmy.blahaj.zone##.comment:has(*:matches-attr(href=/.*@hexbear\.net/))

Edit to add this will also make sure it doesn't remove posts that just have the instance in the title, just posts from the instance.