I am developing a plugin for that I have to black list users, so I need to be display one more dropdown item called Black List inside the Bulk Actions dropdown in the Users page, after the Delete option. But I'm unable to see from where these two actions are coming from and also how to black list a particular user.
My idea is to add one more field is_blacklisted in user table as Boolean with default value
There's a filter, but it's only useful to remove bulk actions.
From this WPSE question, answer and comments, there's the following workaround: add a custom option to the dropdown with jQuery and hook into
admin_action_$your-action to catch the submission.
Your doubt about blocking a user deserves another question, but I'd start a research here first.