I normally have a pretty good experience with Black Cabs, not all, but most. *
I gave way to a black cab swerving over 2 lanes to get to a fare. I got a cheerful thanks and wave which I returned. They're all just trying to make a living.
*but some mini cabs on the other hand.....
+1
if you actually shoulder check and make yourself aware to them you are generally fine. if you ride like a cock or have no bike skills, then fair game.