I believe I just encountered a bug in the privacy rule.
Please have a look at the picture: this type of rule involving a list of users, only works if the list is not empty but there is at least one entry. If the list is empty, in my case, it just didn’t return results when searching.
I tried 1000 ways and nothing worked, this is the only way that worked. But actually I just discovered this bug (I think), so it might work the other way around too, if I add at least one user to each list (blocking and blocked by)
edit: the label “blocked” is just something I wrote quickly while testing a way to make this work.