With the black/mallard debate, I find black ducks are just a whole lot darker. Any sort of hybrid I've seen, as you said, is typically a black duck. I've seen more black ducks with what looks to be a recessive mallard gene, than mallards with a recessive black duck gene. If that is the proper terminology...
They also did a genetic study. A very low percentage of suspected hybrid black/mallard ducks actually were in fact, hybrid. They were mostly 100% black ducks based on the genetic sample. The two outward factors that led them to sampling were the white bars or tips on the speculum and some green in the head. Even WITH the green in the head, they found they were still 100% black duck.
I remember when I first got into waterfowl, I couldn't begin to imagine how you would tell the difference between a hen mallard and black duck. After a year or two, I realized black ducks are in fact very "black" in color. Seeing them against the sun or even on the water on an overcast day, they cast quite the silhouette.
To your point, and probably the same for mottled ducks, they are always going to give you less ducks than more. Being in the eastern flyway/NJ, they are almost always going to notch your black duck or hen mallard limit vs anything else.