1) You should be able to add multiple junk mails to the blacklist at a single point in time.
2) The "Add to Blacklist" option should also run the filter across the current mailbox and then automatically move the email (and matching blacklisted emails) to the junk mail folder.
Junk Mail Option
When using IMAP and mark / move an email to the junk mail, I'd like an mailenable option to automatically blacklist that sender. I appreciate that not everyone would want that, but it would let us quickly reduce the spam where we are supporting multiple domains.