Otto, that's a server-side command.
There's a handy check-box in the options menu:
Press Ctrl+O (default keybind to open options)
Go to the Advanced Tab.
Scroll down until you find the "Gui Options" section.
Near the bottom of the section, there's a check-box that says "Show BL_IDs in player list"
Check the box, then hit the "Done" button.
Now, any time you press F2, the player's BL_ID will show up next to their name.