I pretty much agree with the Twink. The black water is purely visual. Most of the toilets I have had extend down into the tank. You will notice that the water will rise and if enough waste is present, the water will start to touch the bottom of that extention into the tank. When you get there you are full and its time to dump.
I had no grey water sensor on my old Hi-Lo trailer and when I heard the glug glug sound from the tub/shower it meant that the tank was full or the water was pushing into the tub, which was the low point in the grey water system.
'04 Classic 30' S.O.,'03 GMC Savana 2500,'14 Honda CTX 700