B and B stands for black and blue - Thrawn plus Vader.
The squads are normally high activation with one or two main attackers. If you outactivate you can wait for your opponent to finish, move something forward (like an Amanin Scout), swap in Vader, and smash something. Then Thrawn auto wins init and you can swap out or smash some more.
It used to be more dominant but there was a rule change that you only get one activation on the first turn after initiative, rather than two.
Thrawn plus Vader hasn’t seen much play recently. You’re more likely to see Mitt’Thrawn with Cad Bane or Boba Assassin, with Pellaeon and/or Morrigan Corde for support.
None of the v-set Vaders synergise well with Thrawn - they go in different directions - there's one with trooper reserves and one that can only played with followers or force users.
The 2010 GenCon runner up featured Lord Vader with Mitt’Thrawn:
http://www.bloomilk.com/Squad/90791/gimp-impsLord Vader can do masses of damage with Lightsaber Assault, Twin, Opportunist and Overwhelming Force or Sith Rage, but he has more counters than he used to - there’s more Ysalamari and Force Absorb on popular pieces.