Will be interesting to see how they turn out
You are right that Khakis are much lighter as ducklings than your two are (in mentioning Khakis I just meant to say that yours might not be solid coloured but may have a slight pencilled pattern to the feathers, like Khaki and Dark Campbells do). I had two that looked the same colour as your two and they both turned out dark chocolate coloured. They had (and still have) black bills and feet. But you are the one who can see your ducklings up close - it's not quite the same only seeing a photo online Having said that, here is a black duckling for comparison:
