Well they made Black-Purple so Black-Cyan is overdue.
I'm really interested in Black-Magenta, or Black-White (I would fight everyone on the Bastion and Fatman for that).
Black white has to be rarer than white. And whites rare.
Black cyan or Black magenta are my favourites. Magenta hopefully off tfb HM and cyan off tfb normal. But i havnt seen one there yet.