I think maybe I can clear the air some. Saunders, you are talking about when you are on your desktop (out of game) and looking at the Steam friends list, right? The "join game" function here is designed and run by Valve. They offer it to any and all game developers who elect to digitally distribute through Steam, but not all choose to (Activision is one that chose not to). General rule of thumb is if the game uses steam cloud (denoted by a little cloud in the game lists, after the name) then the game will allow you to join friends games from the steam friends list.
For Black Ops, activision uses the steam friends list to populate their own friend list, and then uses their own client for join game and server info functions. You have to be in game to use this. Yes, its messy, I kno...
Hope this helps!
SSG.Braxis*BK*