As far as i could see, this feature does not exist:
Because of the facebook-joining-the-fediverse-drama, i have been asked, whether blocking their
domain would be possible on the user level, which it is not, as far as i could see.
Is something like this planned?
it’s already a feature and has been for many many years
all a user need do is click on the little dropdown on a user card and poof domain blocked
Yes of course, how embarassing, thank you. 
could it be done from the settings, like manually adding a domain to a block list rather than searching for a user of said instance and then block it from there?
Which would be, where I was looking for this. I also think, that this would make the feature easier to use and discover.