I have the same flush for the black water tank as you do and i also have that little sticker that says keep valves open. I don't keep them open. I run some water in there so that the water level rises and cleans the tank pretty good. I have a clear piece that goes on right before the flex hose so I can see when the water coming out of the black tank is clean. Then I just call it a day.
Hope it helps. Just don't leave that valve closed and over fill your black tank.
2003 GMC 2500HD DMAX
2008 Yamaha Raptor 700R
2012 Forest River Shockwave 24FSMX
(2012 nights camped:10)