Currently there is no easy I am aware of how you can completely empty or delete a large amount of white/blacklist items after the migration to 5.0. I would like to see a button to completely empty those lists and maybe check boxes to select multiple entries or all currently shown entries.
I'd support this request, and once such selection tickboxes are implemented (to allow bulk deletion as per this FR), I would suggest to add another "multi-action" feature: add/remove from group(s):
select multiple list entries, and then have a button (same as the "group assignment" dropdown in each row in "group managment > domains") to assign (or remove) the selected rows (i.e. domains) to (or from) one or more groups.
Edit: now opened an extended FR here.