While SE completely forgot the blacklist option on the chat, I figured out this one (do not know if its already been told);
  1. Right click name -> Send tell
  2. It will now say 'Send tell [name]' in console (where [name] is the name of the person you want to BL)
  3. Change that into;

    /blacklist add "[name]"

    (Quotes are needed)
  4. Confirm, done