technically, the 'black' tank(s) is whichever one the commode(s) empty into, regardless of whether any other sinks or showers also empty into it. You'll also notice that typical coach gray and black tanks are each half the capacity of the water tank... I assume the thought is that while drycamping you would more than likely fill both tanks at a similar rate, and they would both, together, hold the total amount from the water tank.
The Turners 2014 Thor Palazzo DP 33.3 35' Bunkhouse
BlueOx, '14 Ford Fiesta, DishHD, KingTailgater2, WifiRanger
50k+miles, 200+ parks, 47 states&Canada provinces, since Aug'14