you have to enter the email for which the blacklist entry should be in the input box, and then press 'add' to add a email to the blacklist of the selected user
e.g. you want to add a blacklist entry 'a@b.c' for user 'user@domain.com'
you enter 'user@domain.com' in the textbox
press add
enter 'a@b.c' in the popup and press ok