Originally Posted by
Rongway
You can only target people by name if the command is /tell; in this case you must
/tell Forename Surname@WorldName message
Targeting people by name is no longer valid for any other slash command. If you want to blist someone by slash command, you must be able to use a target tag on them. Slash commands still work with <t>, <mo>, <tt>, etc. If you cannot find the person so that you can target, mouseover, or target-of-target them, you must find them through one of the social menus/windows and blacklist them via context menu.
This is a poorly designed, but working as intended, feature that they implemented in preparation for the World Visit system.